In Book IV of his Physics, Aristotle discusses the meanings of the word “in”:
(1) As the finger is ‘in’ the hand and generally the part ‘in’ the whole.
(2) As the whole is ‘in’ the parts: for there is no whole over and above the parts.
(3) As man is ‘in’ animal and generally species ‘in’ genus.
(4) As the genus is ‘in’ the species and generally the part of the specific form ‘in’ the definition of the specific form.
(5) As health is ‘in’ the hot and the cold and generally the form ‘in’ the matter.
(6) As the affairs of Greece centre ‘in’ the king, and generally events centre ‘in’ their primary motive agent.
(7) As the existence of a thing centres ‘in its good and generally ‘in’ its end, i.e. in ‘that for the sake of which’ it exists.
(8) In the strictest sense of all, as a thing is ‘in’ a vessel, and generally ‘in’ place.
Since the meaning and usage of prepositions tends to vary somewhat according to language, not all of these meanings are very customary in English. Still, none of the meanings are unintelligible to us.
The eighth sense is said to be the “strictest of all,” in the sense that it is the basic physical reality to which we are comparing the other meanings, just as before in time is the first sense of “before” according to the order of time. In this basic sense of “in”, water might be in a glass, or a man in a house.
But just as we saw that “before by nature” is first by nature, here too one sense of “in” is first by nature, and this is Aristotle’s fifth sense, namely as form is in matter. We can see this by noting that every sense of “in” listed by Aristotle, and in my opinion every other reasonable sense, can be analyzed in terms of an analogy with the presence of form in matter.
What do we mean in the strictest sense, when we say that water is in a glass, or a man in a house? This surely implies that the glass contains the water, and the house the man, namely that the container physically surrounds the thing contained. But what is so interesting about this? Wherever I am, there are things all around me, but I don’t say that I am in those things. I might say that I am in the ring composed of this wall, this chair, this table, the computer on which I am working, and so on, but this would be an ad hoc claim made in order to apparently establish that the statement that one thing is in another simply means that some things are surrounding something else. Also, in the case of the glass, we do not have a problem saying that the water is in the glass, even in the strictest sense, even though the water is not completely surrounded. Likewise the man is in the house even if the windows and the doors are open.
Despite the physical incompleteness of the containment in these situations, however, I would still agree that in this basic physical sense, if one thing is in another, it is contained by that other. But something else is going on even here, before we move to any other sense of “in”. This is the implication that the thing which is in something, fills it or partly fills it, so that the thing would be empty if there were not something in it. A glass without water or something similar is an empty glass, and likewise a house without people is an empty house, especially if no one lives there even at other times, and most especially if there is nothing else, such as furniture, in the house.
This is basically the idea that the thing within forms the container, which is otherwise empty and formless. Thus Genesis speaks of the unformed world as a “formless void,” namely as an empty, formless thing. The order of the days of Genesis signifies the order of matter and form, and the last three days consist in filling the empty world with moving things, filling the sea with fish, the air with birds, and the dry land with animals and men. These things are seen as forming the earth, which would otherwise be unformed, as for example is Mars in its current state.
Each of the cases mentioned by Aristotle can be analyzed in a similar way, being understood by analogy with the presence of form in matter.
The part is in the whole because, as was said in the consideration of whole and part, the part is an aspect of the existence of the whole; something which the whole exists as. In this way it is like form, just as redness is a form of the apple, and likewise an aspect of its existence. The whole is in the parts because the whole is made of the parts as its material cause.
The species is in the genus because the genus is like matter which is formed in a specific way to get the species. The genus is in the species because it is a part of the definition of the species, and thus is in the species as a part in the whole.
The effect is in the efficient or final cause because the cause is like something generic, i.e. material, which is made specific, i.e. formed, by the particular effect which it produces.
One thing is in another, therefore, as form is in matter.