Parts and Parmenides

Much of the difficulty of the topic of the previous post simply results from the difficulty of understanding the idea of part and whole. In our original discussion of these concepts, I noted that in order to be a whole, a thing must be itself, but also in a certain way other things which it simply speaking is not.

There is a temptation to say that this is a contradiction: since we admit that the whole is not its part, it cannot be that part in any way, and therefore it cannot satisfy our definition of a whole. And thus it would be impossible to have a whole and parts.

Parmenides attempts to resolve this problem in a simplistic manner, namely by denying the reality of distinction. Since it is impossible for one thing to be distinct from another, it is impossible to for there to be many things which could be made into a whole. There is only one thing, and consequently no need to make anything out of parts.

A more sophisticated and more common solution is to admit the reality of distinction, but to continue to deny the possibility of forming wholes from parts. By confusing the idea of “fundamental” as the primary material cause with the idea of “fundamental” as most real, Sean Carroll accepts this solution.

Both proposed solutions are contrary to common sense, and both effectively deny the reality of all the things of our common experience. Parmenides makes this denial openly, Carroll by implication, although he at least wishes to avoid it.

There is an additional inconsistency in Carroll’s view insofar as we cannot avoid thinking of the universe as a kind of whole. In other words, just as Parmenides wished to say, “there is only one thing,” Carroll wishes to say, “there are only many things.” But this cannot be done: for there cannot be many things, unless those many are in some way one.

Alexander Pruss attempts to formulate a still more sophisticated solution, which to some extent we have already discussed:

Some people are attracted to nihilism about proper parthood: no entity has proper parts. I used to be rather attracted to that myself, but I am now finding that a different thesis fits better with my intuitions: no entity is (fully) grounded. Or to put it positively: only fundamental entities exist.

This has some of the same consequences that nihilism about proper parthood would. For instance, on nihilism about proper parthood, there are no artifacts, since if there were any, they’d have proper parts. But on nihilism about ontological grounding, we can also argue that there are no artifacts, since the existence of an artifact would be grounded in social and physical facts. Moreover, nihilism about ontological grounding implies nihilism about mereological sum: for the existence of a mereological sum would be grounded in the existence of its proper parts. However, nihilism about ontological grounding is compatible with some things having parts–but they have to be things that go beyond their parts, things whose existence is not grounded in the existence and relations of their parts.

Note that he states that he was formerly attracted to the view that “no entity has proper parts.” This would assert, like the views of Parmenides and Sean Carroll, that wholes and parts are impossible. Since this seems too opposed to common sense, he formulates a new view, where it is possible for a thing to have parts, but the thing must “go beyond” its parts in some way. The existence of the whole “is not grounded” in the existence and relations of its parts.

It is not clear to me precisely what he means by grounding here, and his position could be true, if this is understood in some ways, and not true, if it is understood in others. It could be taken in a fairly tautological sense: something with parts is real if it is really one thing, and not merely many things. But we could just as well say that many things cannot exist without being in some way one. And this does not seem to be Pruss’s intended meaning, since he denies the reality of artifacts, even in this text, which would not be necessary on this understanding.

In any case there does seem to be some remaining desire to deny in some way the possibility of whole and part, indicated for example in the statement that “only fundamental entities exist.” Of course if we understand “fundamental” to mean “real,” then the statement is that only real things exist, and this is obviously true. But like in the case of Carroll, it is evident that fundamental here is meant to refer in some way to what things are made from, namely to material causes. The difference is that rather than saying that the fundamental things are particles of some kind, Pruss would say that some fundamental things are particles, while others are human beings and so on.

Form and the Goodness of God

In his commentary on the Sentences of Peter Lombard, St. Thomas says:

Loquendo autem de necessitate quae est ex suppositione divini propositi, quo propter benevolentiam suae bonitatis voluit unicuique eam communicare secundum suam capacitatem, necessarium est quod cuilibet materiae praeparatae forma infundatur. Non tamen eodem modo est in omnibus formis; quia formae quae mediantibus secundis agentibus in materia producuntur, necessario in materia disposita recipiuntur necessitate conditionata in comparatione ad Deum, cujus virtute cetera agentia agunt; sed necessitate absoluta per comparationem ad agentia proxima, quae necessitate naturae agunt propter ordinem divinitus eis impositum, quem praeterire non possunt. Formae autem quae immediate a Deo inducuntur, non habent necessitatem absolutam ex parte agentis, sed quaedam ex parte recipientis, sicut in perfectionibus quae sunt de esse naturae, ut est anima rationalis. Formae autem quae non debentur naturae, sicut gratia et virtutes, immediate a Deo productae, nihil habent de necessitate absoluta, sed solum de necessitate ex suppositione divini ordinis, ut dictum est.

Which is, more or less:

But speaking of the necessity which is on the supposition of the divine intention, by which on account of the benevolence of his [God’s] goodness, he wished to share with each thing according to its capacity, it is necessary that form be infused into every prepared matter. But nevertheless not in the same way in all forms; since forms which are produced in matter through the mediation of secondary agents are necessarily received into matter with a conditional necessity in comparison with God, by the power of whom other agents act, but with an absolute necessity in comparison to the proximate agents, which act by necessity of nature on account of the order divinely imposed on them, which they cannot omit. But forms which are immediately imposed by God, do not have an absolute necessity on the part of the agent, but a certain necessity on the part of the recipient, as in perfections which are of the being of nature, as is the rational soul. And forms which are not owed to nature, like grace and the virtues, produced immediately by God, have nothing of absolute necessity, but only of the necessity on the supposition of the divine order, as was said.

St. Thomas’s overall point is that every disposed matter receives its corresponding form, and that this happens on account of the goodness of God. He asserts that there are three ways in which this happens:

(1) With respect to forms produced through secondary causes, the things involved cannot naturally fail to produce the form in disposed matter, although this is taken to be conditional in comparison with God, presumably in the sense that if God wanted things to happen otherwise, they could happen otherwise, just as St. Thomas asserts that in the Eucharist accidents can exist without substance, by the divine will.

This first case seems intended to cover all natural changes, with one exception (we will get to the exception in a few moments.) So for example, according to St. Thomas, if you dispose matter to be a certain color, it will necessarily be that color; if you dispose matter to be living matter, it will necessarily be living; and so on. Just as we saw with lines and squares, properly disposed matter always has the corresponding form.

(2) Second, there are forms that are “immediately imposed by God,” but which have a certain natural necessity on the part of the recipient. He mentions the rational soul as though it were an example here, but in fact it seems to be the only thing which is meant to fall into this category rather than the third category.

This happens because St. Thomas considers the human soul to be something exceptional and unique in nature, as we can see from his discussion of the human soul as subsistent:

I answer that, It must necessarily be allowed that the principle of intellectual operation which we call the soul, is a principle both incorporeal and subsistent. For it is clear that by means of the intellect man can have knowledge of all corporeal things. Now whatever knows certain things cannot have any of them in its own nature; because that which is in it naturally would impede the knowledge of anything else. Thus we observe that a sick man’s tongue being vitiated by a feverish and bitter humor, is insensible to anything sweet, and everything seems bitter to it. Therefore, if the intellectual principle contained the nature of a body it would be unable to know all bodies. Now every body has its own determinate nature. Therefore it is impossible for the intellectual principle to be a body. It is likewise impossible for it to understand by means of a bodily organ; since the determinate nature of that organ would impede knowledge of all bodies; as when a certain determinate color is not only in the pupil of the eye, but also in a glass vase, the liquid in the vase seems to be of that same color.

Therefore the intellectual principle which we call the mind or the intellect has an operation “per se” apart from the body. Now only that which subsists can have an operation “per se.” For nothing can operate but what is actual: for which reason we do not say that heat imparts heat, but that what is hot gives heat. We must conclude, therefore, that the human soul, which is called the intellect or the mind, is something incorporeal and subsistent.

Elsewhere he says that the human soul alone is special in this way. Thus in the next article he specifically denies subsistence in regard to the souls of other animals:

I answer that, The ancient philosophers made no distinction between sense and intellect, and referred both a corporeal principle, as has been said. Plato, however, drew a distinction between intellect and sense; yet he referred both to an incorporeal principle, maintaining that sensing, just as understanding, belongs to the soul as such. From this it follows that even the souls of brute animals are subsistent. But Aristotle held that of the operations of the soul, understanding alone is performed without a corporeal organ. On the other hand, sensation and the consequent operations of the sensitive soul are evidently accompanied with change in the body; thus in the act of vision, the pupil of the eye is affected by a reflection of color: and so with the other senses. Hence it is clear that the sensitive soul has no “per se” operation of its own, and that every operation of the sensitive soul belongs to the composite. Wherefore we conclude that as the souls of brute animals have no “per se” operations they are not subsistent. For the operation of anything follows the mode of its being.

As a result of this special account of the human soul, St. Thomas asserts that natural agents do not have the capacity, considered in themselves, to produce a human soul. Consequently he says that there is not an absolute necessity on the part of the agents, meaning that there is no necessity because there is not even the possibility, considered on the part of the agents in themselves. But he says there is a necessity on the part of the recipient, meaning that nature would be lacking its appropriate perfection if e.g. the union of human seed and egg did not result in a human soul. And on account of the goodness of God, such a lack will never be permitted. Consequently every matter that is disposed to be human, will actually be human, just as in the case of the forms of the first kind.

(3) Finally, there are forms which are not “owed” to nature in the way that other natural forms are, and in the way in which even the human soul is owed. Here St. Thomas speaks of grace and “the virtues,” by which he means supernatural virtues, not natural virtues.

In order to understand this case, we can imagine a situation where we construct many shapes: squares, circles, triangles and so on. We paint all of them white, with one exception: whenever we have squares, we paint them red. In essence, we have invented a rule: we consider squares, and no other shapes, disposed to be red, and we cause them actually to be so.

In our example, there is nothing about being square that would actually make a thing red. But there is a certain order there, imposed by ourselves, whereby being square is effectively a disposition to be red. In a similar way, St. Thomas asserts that there are dispositions that God considers to be dispositions for grace, even though in themselves they would not be sufficient to make a thing have grace, or even to make a thing need grace. But St. Thomas says that on account of the goodness of God, he considers “doing as well as one can” to be the disposition for grace, and thus God always gives grace to one who does their best, even if they are merely doing their best on a natural level.

While it is not the point of this post, it is because of this account of God’s goodness in relation to form and disposed matter that St. Thomas is fundamentally opposed to any kind of rigid understanding of the doctrine of extra ecclesiam nulla salus. According to St. Thomas, whether there is a natural necessity for it or not, every disposed matter receives its appropriate form, on account of the goodness of God.

 

Mixing Water and Wine

St. Thomas discusses what happens if you mix consecrated wine with another liquid:

I answer that, The truth of this question is evident from what has been said already. For it was said above (3; 5, ad 2) that the species remaining in this sacrament, as they acquire the manner of being of substance in virtue of the consecration, so likewise do they obtain the mode of acting and of being acted upon, so that they can do or receive whatever their substance could do or receive, were it there present. But it is evident that if the substance of wine were there present, then some other liquid could be mingled with it.

Nevertheless there would be a different effect of such mixing both according to the form and according to the quantity of the liquid. For if sufficient liquid were mixed so as to spread itself all through the wine, then the whole would be a mixed substance. Now what is made up of things mixed is neither of them, but each passes into a third resulting from both: hence it would result that the former wine would remain no longer. But if the liquid added were of another species, for instance, if water were mixed, the species of the wine would be dissolved, and there would be a liquid of another species. But if liquid of the same species were added, of instance, wine with wine, the same species would remain, but the wine would not be the same numerically, as the diversity of the accidents shows: for instance, if one wine were white and the other red.

But if the liquid added were of such minute quantity that it could not permeate the whole, the entire wine would not be mixed, but only part of it, which would not remain the same numerically owing to the blending of extraneous matter: still it would remain the same specifically, not only if a little liquid of the same species were mixed with it, but even if it were of another species, since a drop of water blended with much wine passes into the species of wine (De Gener. i).

Now it is evident that the body and blood of Christ abide in this sacrament so long as the species remain numerically the same, as stated above (4; 76, 6, ad 3); because it is this bread and this wine which is consecrated. Hence, if the liquid of any kind whatsoever added be so much in quantity as to permeate the whole of the consecrated wine, and be mixed with it throughout, the result would be something numerically distinct, and the blood of Christ will remain there no longer. But if the quantity of the liquid added be so slight as not to permeate throughout, but to reach only a part of the species, Christ’s blood will cease to be under that part of the consecrated wine, yet will remain under the rest.

Given the doctrine of transubstantiation, at least as St. Thomas understands it, so that it implies the existence of accidents without a subject, it is very difficult to understand how such a mixing would be possible at all. But his general position here is that a process analogous to substantial change necessarily happens if you mix anything into the consecrated wine, either according to a part of the wine, or according to the whole. He explains this kind of change in article five of the same question:

I answer that, Since “the corruption of one thing is the generation of another” (De Gener. i), something must be generated necessarily from the sacramental species if they be corrupted, as stated above (Article 4); for they are not corrupted in such a way that they disappear altogether, as if reduced to nothing; on the contrary, something sensible manifestly succeeds to them.

Nevertheless, it is difficult to see how anything can be generated from them. For it is quite evident that nothing is generated out of the body and blood of Christ which are truly there, because these are incorruptible. But if the substance, or even the matter, of the bread and wine were to remain in this sacrament, then, as some have maintained, it would be easy to account for this sensible object which succeeds to them. But that supposition is false, as was stated above (75, 2,4,8).

Hence it is that others have said that the things generated have not sprung from the sacramental species, but from the surrounding atmosphere. But this can be shown in many ways to be impossible. In the first place, because when a thing is generated from another, the latter at first appears changed and corrupted; whereas no alteration or corruption appeared previously in the adjacent atmosphere; hence the worms or ashes are not generated therefrom. Secondly, because the nature of the atmosphere is not such as to permit of such things being generated by such alterations. Thirdly, because it is possible for many consecrated hosts to be burned or putrefied; nor would it be possible for an earthen body, large enough to be generated from the atmosphere, unless a great and, in fact, exceedingly sensible condensation of the atmosphere took place. Fourthly, because the same thing can happen to the solid bodies surrounding them, such as iron or stone, which remain entire after the generation of the aforesaid things. Hence this opinion cannot stand, because it is opposed to what is manifest to our senses.

And therefore others have said that the substance of the bread and wine returns during the corruption of the species, and so from the returning substance of the bread and wine, ashes or worms or something of the kind are generated. But this explanation seems an impossible one. First of all, because if the substance of the bread and wine be converted into the body and blood of Christ, as was shown above (75, 2,4), the substance of the bread and wine cannot return, except the body and blood of Christ be again changed back into the substance of bread and wine, which is impossible: thus if air be turned into fire, the air cannot return without the fire being again changed into air. But if the substance of bread or wine be annihilated, it cannot return again, because what lapses into nothing does not return numerically the same. Unless perchance it be said that the said substance returns, because God creates anew another new substance to replace the first. Secondly, this seems to be impossible, because no time can be assigned when the substance of the bread returns. For, from what was said above (4; 76, 6, ad 3), it is evident that while the species of the bread and wine remain, there remain also the body and blood of Christ, which are not present together with the substance of the bread and wine in this sacrament, according to what was stated above (Question 75, Article 2). Hence the substance of the bread and wine cannot return while the sacramental species remain; nor, again, when these species pass away; because then the substance of the bread and wine would be without their proper accidents, which is impossible. Unless perchance it be said that in the last instant of the corruption of the species there returns (not, indeed, the substance of bread and wine, because it is in that very instant that they have the being of the substance generated from the species, but) the matter of the bread and wine; which, matter, properly speaking, would be more correctly described as created anew, than as returning. And in this sense the aforesaid position might be held.

However, since it does not seem reasonable to say that anything takes place miraculously in this sacrament, except in virtue of the consecration itself, which does not imply either creation or return of matter, it seems better to say that in the actual consecration it is miraculously bestowed on the dimensive quantity of the bread and wine to be the subject of subsequent forms. Now this is proper to matter; and therefore as a consequence everything which goes with matter is bestowed on dimensive quantity; and therefore everything which could be generated from the matter of bread or wine, if it were present, can be generated from the aforesaid dimensive quantity of the bread or wine, not, indeed, by a new miracle, but by virtue of the miracle which has already taken place.

This is rather strange, because he seems to be saying that the subsequent substantial forms inhere in quantity as in a subject, and that there is no matter there. But if this is possible in any way, and in particular if things remain in this state permanently, as he seems to suggest, then there seems to be little reason not to adopt Descartes’s view of material substance in general, and say that quantity is always the subject of substantial forms, rather than saying that some parts of the world have matter as a subject, and other parts quantity. The account might be more reasonable if he were to accept that when a new substance is generated, matter again comes to be, not by being “created anew,” but because the being of matter in general is from substantial form.

As we can see, this discussion is especially complex on account of the doctrine of transubstantiation and St. Thomas’s account of that doctrine. But if we simply consider the mixing of two liquids in general, various difficulties will remain. Suppose we have a glass of water and a glass of wine, and mix the two together. What exactly will happen?

It is manifest to the senses that when we do this, there is a period of time when parts of the resulting liquid are water, just as it was, and parts are wine, just as it was, without any mixture. But what about the surface where the two are in contact? What is happening there?

According to St. Thomas, there will be a quantitative part which shares in the qualities of each. And this is pretty reasonable. Just as we can see that part is wine and part is water, at a certain point we can see that part is watery wine. But how exactly did that watery part get that way? If it is a certain size, was there a sudden transition of a part which was water into the watery wine? Or the like with the wine becoming watery? Or was there a continuous process with an expanding mixed region? The last possibility seems most consistent with what we see, but it might be difficult to analyze this in terms of substantial change, as St. Thomas does, because such a continuous process would have no first moment when the mixed substance came to be. For if it did, it would come to be with a definite size, and thus the process would not be continuous, but would imply that some part suddenly went from not being watery wine to being watery wine.

Of course, it is one thing to say there are difficulties. It is quite another to say that they mean that the thing cannot happen. So none of this proves that the mixing of liquids is not a substantial change. Nonetheless, many of the ancient naturalists were moved by such considerations to adopt some form of atomic theory. If water and wine are each composed of atoms, the mixing process is easily understood — it is simply the movement in place of these atoms. Each part of the water remains as it was even qualitatively, and likewise each part of the wine, but the resulting mixture has different sensible qualities because one cannot distinguish the diverse qualities of each, just as mixing two very fine sands of different color may appear to result in a third color, even though the grains of sand are not changing qualitatively.

Modern atomic theory, of course, has far stronger arguments for it, but they are in principle, or at least were in the 18th and 19th centuries, of a very similar kind: atomic theory simply does a good job of explaining many of the things that we see happen in the world.

This is closely related to the discussion in the last post. When we construct a bicycle out of parts, it is manifest to the senses that the parts look just like they did before they were parts. And this is necessary, if it is true that those parts are governed by the same natural laws after they become parts that they were before they became parts. For however the parts “look,” they look this way because of how they act on the senses. So if their action does not change, the “way they look” will not change. Similarly, when we mix liquids, if the water parts and the wine parts do not change how they behave, the account one gives of the mixture must be an atomic theory or something very like it. That is, there must remain very small parts that act like water, and very small parts that act like wine. Or, given that wine and water are not in fact elements, at least the basic elemental parts must continue to act like those elemental parts.

Substance

Aristotle says in his Categories “Substance, in the truest and primary and most definite sense of the word, is that which is neither predicable of a subject nor present in a subject.” Earlier in the text, he explains these characteristics:

Of things themselves some are predicable of a subject, and are never present in a subject. Thus ‘man’ is predicable of the individual man, and is never present in a subject.

By being ‘present in a subject’ I do not mean present as parts are present in a whole, but being incapable of existence apart from the said subject.

Some things, again, are present in a subject, but are never predicable of a subject. For instance, a certain point of grammatical knowledge is present in the mind, but is not predicable of any subject; or again, a certain whiteness may be present in the body (for colour requires a material basis), yet it is never predicable of anything.

Other things, again, are both predicable of a subject and present in a subject. Thus while knowledge is present in the human mind, it is predicable of grammar.

There is, lastly, a class of things which are neither present in a subject nor predicable of a subject, such as the individual man or the individual horse. But, to speak more generally, that which is individual and has the character of a unit is never predicable of a subject. Yet in some cases there is nothing to prevent such being present in a subject. Thus a certain point of grammatical knowledge is present in a subject.

It is not clear from this how Aristotle would deal with sentences like, “That man is Socrates,” where something individual seems to be predicated of a subject. Nonetheless, we can at least see what he means: things which are “predicable of a subject” refer to common terms and ideas, while individual realities are not said to be predicable of a subject. And something is “present in a subject” when it is a quality, property, or attribute of a thing. So by substance he refers to individual realities which are not qualities or attributes of other things.

Why does he say, “By being ‘present in a subject’ I do not mean present as parts are present in a whole, but being incapable of existence apart from the said subject”? Later in the text he explains this:

The fact that the parts of substances appear to be present in the whole, as in a subject, should not make us apprehensive lest we should have to admit that such parts are not substances: for in explaining the phrase ‘being present in a subject’, we stated’ that we meant ‘otherwise than as parts in a whole’.

In other words, he does not want to say that my arms and legs are “not substances.” Not because they are individual things in the way that I myself am an individual thing, but because neither do they seem to be merely qualities that I happen to have. The word “substance” itself is taken from the idea of “standing under,” and this is related to his general definition. Substances are the things that have everything else predicated of them or present in them, and in this sense they “stand under” those other things. And my arm seems to be a part of what stands under on an approximately equal level: my arm can be colored just as I can be colored, and so on.

Someone could object: if Socrates is a white man, then isn’t “white” a part of the whole, namely the “white man”? If this is the case, the Aristotle has no basis for his distinction between substantial parts and other things which are present in a subject.

But it is not difficult to respond to this. I tried to give a formal explanation of the idea of whole and part earlier. And as I said there, the whole must be distinct from each of the parts. I am not my legs, my arms, my torso, or my head. But in the case of Socrates, he is both the man and the white man. So “man” cannot be a part of the whole “white man”, since the man actually is the white man.

Nonetheless, it is possible to reformulate the objection to make it more problematic. Instead of suggesting that “white” and “man” are two parts of a whole, we could say that “whiteness” and “individual humanity” (or possibly “individuality” and “humanity”) are parts of Socrates. Socrates is not any of these things as such, and consequently we cannot refute the idea that they are parts as we did in the previous case.

I doubt that there can be any formal refutation of this idea, because if we could, we could refute the definition, “man is a rational animal,” in a similar way. In fact, if we add nothing to the idea of part and whole as we have defined them, it is not even false that “whiteness” and “humanity” can be parts of a whole. They can in fact be parts of a whole. It is just not the kind of whole that we are interested in when we ask the question, “what is that thing?” And this is the question that Aristotle is interested in when he speaks of substance, and consequently that kind of whole and part is not the kind he spoke about when he said that being “present in” a subject means “otherwise than as a part.”

 

Spinoza’s Geometrical Ethics

Benedict Spinoza, admiring the certainty of geometry, writes his Ethics Demonstrated in Geometrical Order in a manner imitating that of Euclid’s Elements.

Omitting his definitions and axioms for the moment, we can look at his proofs. Thus we have the first:

1: A substance is prior in nature to its states. This is evident from D3 and D5.

The two definitions are of “substance” and “mode,” which latter he equates with “state of a substance.” However, neither definition explains “prior in nature,” nor is this found in any of the other definitions and axioms.

Thus his argument does not follow. But we can grant that the claim is fairly reasonable in any case, and would follow according to many reasonable definitions of “prior in nature,” and according to reasonable axioms.

He proceeds to his second proof:

2: Two substances having different attributes have nothing in common with one another. This is also evident from D3. For each ·substance· must be in itself and be conceived through itself, which is to say that the concept of the one doesn’t involve the concept of the other.

D3 and D4 (which must be used here although he does not cite it explicitly in the proof) say:

D3: By ‘substance’ I understand: what is in itself and is conceived through itself, i.e. that whose concept doesn’t have to be formed out of the concept of something else. D4: By ‘attribute’ I understand: what the intellect perceives of a substance as constituting its essence.

Thus when he speaks of “substances having different attributes,” he means ones which are intellectually perceived as being different in their essence.

Once again, however, “have nothing in common” is not found in his definitions. However, it occurs once in his axioms, namely in A5:

A5: If two things have nothing in common, they can’t be understood through one another—that is, the concept of one doesn’t involve the concept of the other.

The axiom is pretty reasonable, at least taken in a certain way. If there is no idea common to the ideas of two things, the idea of one won’t be included in the idea of the other. But Spinoza is attempting to draw the conclusion that “if two substances have different attributes, i.e. are different in essence, then they have nothing in common.” But this does not seem to follow from a reasonable understanding of D3 and D4, nor from the definitions together with the axioms. “Dog” and “cat” might be substances, and the idea of dog does not include that of cat, nor cat the idea of dog, but they have “animal” in common. So his conclusion is not evident from the definition, nor does it follow logically from his definitions and axioms, nor does it seem to be true.

And this is only the second supposed proof out of 36 in part 1 of his book.

I would suggest that there are at least two problems with his whole project. First, Spinoza knows where he wants to get, and it is not somewhere good. Among other things, he is aiming for proposition 14:

14: God is the only substance that can exist or be conceived.

This is closely related to proposition 2, since if it is true that two different things can have nothing in common, then it is impossible for more than one thing to exist, since otherwise existence would be something in common to various things.

Proposition 14 is absolutely false taken in any reasonable way. Consequently, since Spinoza is absolutely determined to arrive at a false proposition, he will necessarily employ falsehoods or logical mistakes along the way.

There is a second problem with his project. Geometry speaks about a very limited portion of reality. For this reason it is possible to come to most of its conclusions using a limited variety of definitions and axioms. But ethics and metaphysics, the latter of which is the actual topic of his first book, are much wider in scope. Consequently, if you want to say much that is relevant about them, it is impossible in principle to proceed from a small number of axioms and definitions. A small number of axioms and definitions will necessarily include only a small number of terms, and speaking about ethics and metaphysics requires a large number of terms. For example, suppose I wanted to prove everything on this blog using the method of definitions and axioms. Since I have probably used thousands of terms, hundreds or thousands of definitions and axioms would be required. There would simply be no other way to get the desired conclusions. In a similar way, we saw even in the first few proofs that Spinoza has a similar problem; he wants to speak about a very broad subject, but he wants to start with just a few definitions and axioms.

And if you do employ hundreds of axioms, of course, there is very little chance that anyone is going to grant all of them. They will at least argue that some of them might be mistaken, and thus your proofs will lose the complete certainty that you were looking for from the geometrical method.