There are two key general premises in the fourth way: “what is more and less is such by its relation to what is most” and “what is most such in a genus is the cause of all in the genus”. St. Thomas holds that both axioms have application in physical science, though the example he gives of their application is incorrect – namely that things are hot because they are mixed with fire, just as cookies are sweet because they are made with sugar. While we must abandon this particular example, it is still important to understand the axioms of the fourth way in such a way as to preserve their application to physical science. We can understand them in this way if we see them as speaking to the existence of an exemplar cause which is taken for granted as existing whenever we try to give an explanation of some reality that is common to many things, and, in the case when some reality is more or less, provides us with an insight as to what exactly makes certain realities exist in degrees.
The inquiry we are making here into natural science is one that seeks something “fact- like” and not “law- like”, that is, we are seeking not to derive a conclusion from observations but rather to speak about the criterion by which we will judge various possible observations and judge certain explanations to be dead ends. The question is this: what would count as an explanation of things that admit of degrees? This much seems clear: we’d have to toss out all explanations that appeal to something that has the thing that admits of degrees by the action of another and not by themselves. Things on earth might be more or less stable, but one can’t explain the stability by invoking turtles for the earth to rest on. But what about sharpness or aerodynamic form? Knives and cars have these things of themselves, and so it is not a matter of looking for some other thing by which a knife is sharp or the car is aerodynamic – but in both cases there is an intelligible reality, which simply waits to be known, which provides a measure that illuminates exactly why each of these things can admit of more and less. This does not mean that there is, say, one and only one perfectly aerodynamic shape, but rather that one can find a single equation for the drag that we seek to overcome by various shapes. One misses the point if he critiques St. Thomas’s axiom by saying “just because things are more or less aerodynamic doesn’t mean that there exists something perfectly aerodynamic”. What is “most such” here is the drag equation so far as it articulates a single reality in nature and is actually used to discern what is more or less of some kind. The sort of causality that the equation has when used in this way is exemplar causality.
We stress the objective existence of the exemplar cause. The exemplar is what we are seeking for when we seek to answer the question “what is this X that we see can be more or less in things?” This is some intelligible reality which we take for granted in any investigation – which is in fact a given prior to the investigation being possible. Though objective, however, the exemplar of some multitude will not alaways have the same existence, since the existence of various things that can be more or less is variously tied up with finite existence, matter, and other such things. In the examples we gave above, all of the exemplar causes were of things that required matter and finite existence; for example, the whole existence of drag is tied up with pressure, mass density, velocity, etc. and the whole existence of sharpness is tied up with thinness, rigidity, pressure, etc. But what do we do with a notion like good, which is not only distinguished from the finite, but even in some way opposed to it, since we can’t love any good so far as it does not give us something desirable, and every finite good must be such? Here the exemplar cannot be given among the finite.
Just what sort of existence this exemplar has is not always clear – and the diverse answers to the question serve to divide the schools of Plato and Aristotle. Nevertheless, there is still an agreement between them about the objectivity of the exemplar. Setting aside the particular controversies, it is enough for us to say that the exemplar we seek is not an inquiry into the structure of our own minds. Among the things of natural science, the exemplar is clearly bound up with matter, though it is not exactly the same thing as the various concrete individuals. The exemplar of things like “goodness, truth, dignity and other such things” does not and cannot have this sort of being, for the reason given above.