Commentary Aristotle Posterior Analytics
Lecture 42
Science is not concerned with things caused by fortune or with things learned through sense-perception a5. There is no knowledge by demonstration of chance conjunctions; for chance conjunctions exist neither by necessity nor as general connexions but comprise what comes to be as something distinct from these. Now, demonstration is concerned only with one or other of these two; for all reasoning proceeds from necessary or general premises, the conclusion being necessary if the premises are necessary and general if the premises are general. Consequently, if chance conjunctions are neither general nor necessary, they are not demonstrable.
b28. Scientific knowledge is not possible through the act of perception. Even if perception as a faculty is of ‘the such’ and not merely of a ‘this somewhat’, yet one must at any rate actually perceive a ‘this somewhat’, and at a definite present place and time: but that which is commensurately universal and true in all cases one cannot perceive, since it is not ‘this’ and it is not ‘now’; if it were, it would not be commensurately universal — the term we apply to what is always and everywhere. Seeing, therefore, that demonstrations are commensurately universal and universals imperceptible, we clearly cannot obtain scientific knowledge by the act of perception:
b34. nay, it is obvious that even if it were possible to perceive that a triangle has its angles equal to two right angles, we should still be looking for a demonstration—we should not (as some say) possess knowledge of it; for perception must be of a particular, whereas scientific knowledge involves the recognition of the commensurate universal. So if we were on the moon, and saw the earth shutting out the sun’s light, we should not know the cause of the eclipse: we should perceive the present fact of the eclipse, but not the reasoned fact at all, since the act of perception is not of the commensurate universal. I do not, of course, deny that by watching the frequent recurrence of this event we might, after tracking the commensurate universal, possess a demonstration, for the commensurate universal is elicited from the several groups of singulars.
The commensurate universal is precious because it makes clear the cause; so that in the case of facts like these which have a cause other than themselves universal knowledge is more precious than sense-perceptions and than intuition. (As regards primary truths there is of course a different account to be given.)
Hence it is clear that knowledge of things demonstrable cannot be acquired by perception, unless the term perception is applied to the possession of scientific knowledge through demonstration.
a11. Nevertheless certain points do arise with regard to connexions to be proved which are referred for their explanation to a failure in sense-perception: there are cases when an act of vision would terminate our inquiry, not because in seeing we should be knowing, but because we should have elicited the universal from seeing; if, for example, we saw the pores in the glass and the light passing through, the reason of the kindling would be clear to us because we should at the same time see it in each instance and intuit that it must be so in all instances.
There is no knowledge by demonstration. After assigning the basis of unity and of diversity of sciences on the part of the generic subject and on the part of the principles, the Philosopher here continues with each.
First, with the subjects about which there is science.
Second, with the principles, at: all syllogisms cannot have (89a18; [382]).
In regard to the first it should be noted that above he laid down two marks of the genus which is the subject of the science: one of these is that it be composed of first principles, and the other is that its parts and properties be per se. Now, one of these conditions is lacking in things that happen by fortune, because they do not come about per se, but per accidens and beyond one’s intention, as is proved in Physics II. The other mark is lacking in things that are known through sense, which are first in our knowledge.
Therefore, he shows first that science is not concerned with things which come about by fortune.
Second, that it is not of things that are known through sense, at: scientific knowledge is not possible (87b28; [375]).
He says therefore first (87b19) that demonstrative science cannot be concerned with things that occur through fortune, that is, things that come about per accidens. He proves this in the following way: Every demonstrative syllogism proceeds either from necessary propositions or from propositions which are true for the most part. Now, from necessary propositions a necessary conclusion follows, as has been proved above; in like manner, from propositions which are true for the most part, a conclusion follows which is true for the most part, or perhaps one which is necessary, in the sense that the necessary can follow from the contingent, as the true can follow from the false. However, it never happens that from propositions which are true for the most part, a conclusion will follow which is true in few cases; because that would mean that in certain cases the propositions would be true and the conclusion false, which is impossible, as has been shown. It is necessary, therefore, that the conclusion of a demonstrative syllogism either be necessary or be true in the majority of cases. But that which is by fortune is neither necessary nor is it true in the majority of cases, but it occurs rarely, as is proved in Physics II. Therefore, demonstrative science cannot be concerned with that which is by fortune.
It should be noted, however, that there happens to be demonstration of things which occur, as it were, for the most part, insofar as there is in them something of necessity. But the necessary, as it is stated in Physics II, is not the same in natural things (which are true for the most part and fail to be true in a few cases) as in the disciplines, that is, in mathematical things, which are always true. For in the disciplines there is a priori necessity, whereas in natural science there is a posteriori (which nevertheless is prior according to nature), namely, from the end and form. Hence Aristotle teaches there that to show a 'propter quid', such as if this has to be, say that if an olive is to be generated, it is necessary that this, namely, the olive seed, preexist, but not that an olive is generated of necessity from a given olive seed, because generation can be hindered by some defect. Hence if a demonstration is formed from that which is prior in generation, it does not conclude with necessity, unless perhaps we take as necessary the fact that an olive seed is frequently generative of an olive, because it does this according to a property of its nature, unless it is impeded.
Then, at: scientific knowledge is not possible (87b28) he shows that science is not concerned with things that are known according to sense.
About this he does two things.
First, he shows that science does not consist in sensing.
Second, how sense is ordained to science, at: nevertheless certain points do arise (89a11; [381]).
Concerning the first he does two things.
First, he shows that science is not through sense.
Second, he ranks science above sense, at: the commensurate universal (88a5; [379]).
Concerning the first he does two things.
First, he shows the truth.
Second, he shows the errors of certain ones, at: nay, it is obvious (88b34; [378]).
He says therefore first (88a5) that just as science is not of things which are by fortune, so neither does it consist in knowledge which is through sense. And he proves this in the following way: It is obvious that sense knows the qualities of a thing and not its substance. For the per se object of sense is not the substance and the essence, but some sensible quality, such as hot, cold, white, black, and so on. But qualities of this sort affect singular substances existing in a definite place and time: hence it is necessary that this which is sensed be a this something, that is, a singular substance, and that it be somewhere now, that is, in a definite time and place. From this it is clear that that which is universal cannot fall under sense. For that which is universal is not restricted to the here and now, because then it would not be universal. For we call that a universal which is always and everywhere.
However, this is not to be understood according to the way of an affirmation, as though it pertained to the very notion of a universal or of that which is universal, namely, that it be always and everywhere. For if to exist always and everywhere pertained to the very notion of that which is universal, say to the notion of man or animal, it would then be required that each individual man and animal be always and everywhere, because the notion of man and of animal is found in each of its singulars. Or if this were required by the very notion of the universal, as it is required of a genus that it contain species under it, it would follow that nothing would be universal, if it were not found everywhere and always. According to this, “olive” would not be universal, because it is not found in all lands.
Hence, the statement under consideration must be understood after the manner of a negation or abstraction, that is, that the universal abstracts from every definite time and place. Hence of itself, just as it is found in each thing in one place or time, so it is apt to be found in all.
Thus, therefore, it is clear that the universal does not fall under sense. Therefore, because demonstrations are mainly universal, as has been shown above, it is obvious that science acquired by demonstration does not consist in knowledge obtained through sense.
Then, at: nay, it is obvious (88b34) he excludes the errors of those who believed that science consists in sense-perception.
And this notion seems to belong to those who did not believe that understanding differs from sense and, consequently, that there is no knowledge except sense-knowledge, as it is stated in On the Soul III and Metaphysics IV.
So, to exclude this he says that even if we could perceive through sense that a triangle has three angles equal to two right angles, it would still be necessary to learn this by demonstration if one is to have science; for we do not scientifically know through sense, because sense bears on singulars, whereas science consists in knowing the universal, as has been shown. But because he had given the example of something that cannot be perceived through sense, he makes his point clearer with an example taken from things that can be sensed, namely, from an eclipse of the moon which is caused by the opposition of the earth, which is interposed between the sun and moon, so that the brightness of the sun cannot reach the moon because of the earth’s shadow; and the moon is eclipsed when it enters the shadow. Let us suppose, then, that someone were on the moon and with his senses could perceive the shadow caused by the interposition of the earth. This person, then, would sensibly perceive that the moon is darkened by the shadow of the earth, but this would not mean that he would therefore know totally the cause of the eclipse. For the per se cause of an eclipse is that which universally causes an eclipse. But the universal is not known through sense; rather, we receive universal knowledge from many individually observed cases in which the same thing is found to happen. And so it is through the universal cause that we demonstrate something in the universal way that constitutes science.
Then, at: the commensurate universal (88b34) he shows that science is nobler than sense.
For it is clear that knowledge through the cause is more noble; but the per se cause is the universal cause, as has been said. Therefore, knowledge through the universal cause, such as science is, is more honorable. But because it is impossible to apprehend such a universal cause through sense, it follows that science, which shows the universal cause, is not only more honorable than any sense knowledge, but more honorable than any other intellectual knowledge, provided it concern things which have a cause: for to know something through the universal cause is more noble than any other way of knowing that which has a cause, without knowing the cause. However, the case is otherwise in regard to first things, which do not have a cause. For these are understood in virtue of themselves; and such knowledge of these things is more certain than any science, because it is from such knowledge that science acquires its certitude.
Finally, he concludes to his chief point, namely, that it is impossible through sense to know something demonstrable, unless perhaps one takes “sense” in an equivocal meaning, calling demonstrative science “sense” on the ground that demonstrative science concerns the determinately one just as sense does: on which account sure calculations are called science.
Then, at: nevertheless certain points (89a11) he shows how sense is ordained to science.
For some problematic doubts are caused by the shortcomings of sense. For there would be no need to investigate certain matters, if we were to see them; not because science consists in seeing, but because from things seen by way of experiment the universal is obtained, concerning which there is science. For example, if we should see a porous pane of glass and observed how the light passes through the openings in the glass, we would have 'the scientific answer' as to why the glass is transparent. (He uses this example because it is the opinion of those who supposed that light is a body and that certain bodies are transparent because of openings called pores. But because they cannot be perceived by sight, since they are so small, we wonder why glass is transparent.) A similar example can be given of certain things that have a latent sensible cause. Finally, because he had said that science of such things is not obtained by seeing, he shows that this is true. For in seeing we know isolated singulars; but to know scientifically we must know them all at once in the universal, if we are to know that such and such is the fact in all cases. For we see each individual glass singly, but we receive science that it is thus for all glass.