The principles Kant lays down in the Transcendental Aesthetic are at the foundation of his critique of metaphysics, but I’m struck that Kant (IMO) spends far less time arguing for what is most pertinent to refuting metaphysics. Kant continually returns to the thorny question of whether and in what sense space and time are real, but he says less about how what is given in space limits what can be known.
Kant’s fundamental argument that our mind is limited to objects known in space seems to be this :
By means of outer sense, a property of our mind, we represent to ourselves objects as outside us, and all without exception in space.
Thus “to be an object” requires “to be spacial”. The “without exception” is a limiting condition. Here we have an appeal to a real, bedrock, interior experience, a claim that one must either take or leave and upon which one can certainly build a solid philosophy.
St. Thomas certainly agrees with some sense of what Kant is saying (who couldn’t?) but he continually insists on a qualification that muddles the simplicity of what Kant claims. For St. Thomas, the human person, by intellect, intuits being as spacio-temporal. The standard Thomist formula for this was first given by Cajetan: the proper object of the intellect is being as concretized in a sensible quiddity; or being that is given (as Bergson would put it) “in the undifferentiated solid”. But the contemporary reader gets the best view of what St. Thomas meant in the phrase “being as spacio-temporal”. Thus in one sense all that is given is given in space and time, but it is given precisely as a mode of being, and being can never be considered as purely material in the concept, as though the entire formality of being as spacio-temporal was taken on the side of its spacio-temporal concretion or mode.
The Thomist thus posits a dialectical tension in the proper object of the human intellect: on the one hand it is the case that being only appears to him as concretized in the spacio-temporal order, but this concretion cannot exhaust the possible objects of his intelligence because a.) being, even as known, is essentially formal and so cannot be taken as purely material, even when it is put under the intelligible formality of spacetime, and b.) by analysis man finds himself not only incapable of reducing his notion of being to the spacio-temporal but he is led, in a positive way, to see the purely conditioned nature of spacetime and the openness to the absolute in being. Kant recognizes that the spacio-temoral order is conditioned and not absolute (see sections VIII and IX of the second division of the Transcendental Dialectic), but since he does not have a notion that the intellect properly knows being he does not have an intuition of anything that could be an unconditioned cause of the spacio-temporal order. He concludes, in good logic, that one simply cannot conclude to any cause or even object outside this order, since he has no intuition of what such an object might be.
The contrary of the Kantian analysis is most striking in St. Thomas’s Fourth Way, which presupposes that in the finite experience of good, truth, and “other such things” one intuits and experiences a form which of itself has no limitation, even though that form is completely given in the finite and conditioned order. In concrete terms, when we experience goods, we experience something which is a.) totally finite but b.) which we know we are not considering good because of that finite existence. This is the experience of the real possibility of infinite goodness, or of goodness as infinite. In this possibility, we can know either that such must be, or that some agent has within himself an infinite goodness which he can bring forth. Either way, goodness is simply infinite, measuring all, and all that we experience as limited in spacetime is good by taking part in it, and being measured by it. St. Thomas might even put it in simpler terms: the comparative more or less is most properly in compared to the superlative, and finite goodness, as finite, is comparative.