It’s easy to object that someone is really making a metaphysical claim when they claim to be making a scientific one – but what exactly does this mean?
Minimally, one might mean that someone is making a claim that is not experimentally verifiable or reducible to physical laws. To leave it at this, however, tells us nothing about metaphysics, since the pure negation of verifiability tells us nothing positive. Informing me that a doctrine contains non-verifiable statements tells me about as much as informing me that a box contains several objects that are not snow. What do I know about the contents of either after hearing this?
One might say that metaphysics studies being as such while all other sciences treat of being under some restriction. This explanation has some value – scientific journals after all don’t publish many studies on being or existence, and no one seems to bother looking for “laws of being” - and so there certainly seems to be something to distinguishing science and metaphysics in this way. The difficulty is that this does not seem to be a way in which anyone confuses metaphysics and science.
The challenge thus becomes how to account for metaphysics as non-empirical in a way that ties it somehow to being as such. We can do this by considering the lights of judgment, that is, the various lights that constitute the evidence of diverse sciences.
All knowledge begins in exterior sensation and involves the intellect and interior senses, but each of these three is a distinct mode in our single act of knowing, and so we can judge a single thing known according to three diverse modes in our single cognitive act. Said another way, just because our experience arises from sense it does not follow that we must judge that experience in light of what we know from the exterior senses as such, since this is not the only light in virtue of which we can consider the experience.
As St. Thomas explains it, natural science (which we call just “science”) is that category of experience that arises from exterior sense and is judged by the light of the data furnished by the exterior senses themselves; metaphysics is the category of experience that arises from the exterior sense and is judged by the light of the intellect. By empirical we mean that experience is to be judged according to the way they are known by the exterior senses as such. Said another way, in order for something to count as evidence in natural science, it has to be given in experience according to the way experience is in the exterior senses. All evidence requires some light under which a thing appears as evidence, and that light in natural science is the exterior senses. Metaphysical evidence is not manifested by the same light, and so is not empirical, since it judges experience as known by the intellect. And because what is first known by the intellect is being, ti follows that metaphysics is non-empirical and about being for one and the same reason, namely, the proper object of the intellect as intellect.
Understanding the diverse lights of judgment or evidence helps us divide diverse concepts that are easy to muddle if we are not careful:
1.) Species. For the metaphysician, a species is the proper intelligible nature of some reality. This intelligible nature is discerned according to the way it is present in the intellect: that is, as first known generally and then more distinctly by way of precision in the general concept. Metaphysics is, on the consideration of species, very close to logic and makes much use of it, but the two remain distinct since metaphysics never takes experience as known (or being as known) as the subject that it studies. Metaphysics remains about being as being, logic about the more restricted notion of being as known. Natural science, on the other hand, is not primarily concerned with species according to their intelligible structure, but in the way in which they can be given in the exterior senses as such. This makes species principally a multiplicity of individuals taken as principally multiple. Species thus means principally a population or multitude of traits.
2.) Causes. In sensation, a cause presupposes some actual thing that it must act upon, and thus a cause must always depend on some other in order to exercise its causal power. But as present in the intellect, cause is formally distinguished from dependence, since dependence is a way of being an effect of another. Thus, creation (that is, causality that presupposes no subject) is a possibility in metaphysics whereas it is not a possibility for natural science.
3.) Motion. In the way it is given to sense, motion is the most knowable and obvious of things. The natural scientist thus takes motion as a primarily known reality that is perfectly evident in itself. When judged in light of the intellect, however, motion is the least known and even least knowable of things. Motion is a tremendous problem for the one who judges experience by intellect, for the first thing known by the intellect is being whereas motion as such is becoming, which is a the same time essentially related to being and yet opposed to it. Motion will eventually require the metaphysician to make a division within reality as such into more primordial concepts; whereas the physicist resolves all other realities to the given reality of motion.
4.) Quantity. The natural scientist resolves all thing to experience as given in the exterior senses, but the primary and foundational reality in this order is quantity. All natural science, therefore, progresses by becoming more quantitative. Metaphysics, on the other hand, progresses more by its transcendence of the quantitative order, by progressing towards spiritual reality.
Besides these four, there are all the differences that were treated of in the body of the post: there are distict senses of “evidence”, “judgment”, “experience”, “known by sensation”, etc.
And I left off the whole universe of things that provide their evidence from the light of the interior senses – that is, mathematical reality. Mathematicians don’t wade into the dispute about science and metaphysics, though they could. But there really is a whole universe of mathematical reality out there – even though the mathematicians always seem to want to keep it to themselves.