What is death? Now, “death” is a word which has senses—and each of these senses have related and similar but ultimately differing meanings. When we die, we do not die in the same sense as animals or plants die—for these things die univocally in relation to each other, but we die equivocally in relation to them. That this is true can be shown in the consideration of the following facts and arguments regarding the three metaphysical categories of life:
In vegetative life—that is life capable of growth, healing, and being nourished—we see that their essence, which is a constituent of their substance, is wholly reliant on their material principal, or the matter of these things, for existence; to be nourished requires the ability to metabolize (which requires matter), to heal to have a body (which is material), and to grow—again—a body (which is material). Hence, their essences only have a real existence insofar as they are expressed in their material principles. So, when their material principle ceases to perform these functions—which is what we call physical death—their essence no longer posits anything in reality; their death is absolute—their existence obliterated.
The same follows for those things which are of a purely sensitive nature. Now, sensitive life is life which possesses all of the powers of vegetative life, but also the additional power of perception through the senses (and to recall and react to those perceptions). But of course perception via the senses is reliant on their material subjects—so when we loose our eyes, we loose the sense of sight, and henceforth sight perception. Moreover, the recollections of the perceptions themselves have a material subject, which are the electrical impulses in the brain kept from the perception [1, 2]—a physical snapshot, if you will, of a perception’s result (this Aquinas calls a phantasm ). This too, however, being material, dies with the body. hence, when the material subjects of sensation or recollection are removed, the corresponding essence to that sense and power no longer posit anything in reality. Therefore, nothing remains of the essence of sensitive life upon physical death, because the subjects of perception and recollection (which posit into reality the definition of a sensitive lifeform) are lost; the death is absolute—the sensitive thing has been obliterated.
But in corporeal rational life we find the sensitive powers with the addition of cognition. Now, what is meant by this? What we mean by cognition is abstracted understanding—mental processing which goes beyond the phantasms themselves. For example, there is no phantasm which corresponds to being, but nonetheless we are aware that those things we perceive do have being. Likewise we perceive teleological perfection—purpose fulfillment—yet again for this there are no corresponding phantasms. Yes, there are phantasms of things achieving such state, which can be said to represent the concept of teleological perfection, but in order to represent something, the intellect must already know what it is that it is trying to represent, and therefore the knowledge of the concept is not in the phantasm itself, because representation presupposes that which is being represented, and therefore a representative phantasm is preceded by the knowledge of concept it represents. Thus, as the objects of cognition are immaterial abstractions, Occam’s razor begs us to assume that, because the subjects of all the other powers were of the same type of existence as their objects, the subject of cognition is immaterial. Hence, the subject of cognition, being immaterial, is separate from the material body, and thus is not corrupted by the corruption and passing of the body. And so the corresponding essence of the rational nature in things which are rational do not depend on the body for subsistence. This of course means that rational things survive physical death (otherwise would imply that cognition has a material subject, which we have reasoned against), albeit in an ontologically deprived state (which, I—for one—am glad about; it’s better than simply winking out of existence).
It is for this reason (among others) that mankind’s journey does not end in solely the immaterial heavenly or hellish states, but also in the material equivalents via the Second Coming, during which the physically dead will have given to them new incorruptible bodies, from then on abiding in a final state both fitting to them materially and immaterially —existing in an ontologically restored manner, insofar as their free will allows (lucky for us!).
Such is sufficient for elaborating on the equivocal manner in which we die, and on the human soul’s survival of our physical deaths.
 Aquinas. ST I. Q85. A1. trans. Fr’s OP
 CCC 681–682, 988–990