Human Nature

[Benedict]

I was reading through the Catechism the other day and came across the following:

[b]365 [/b][i]The unity of soul and body is so profound that one has to consider the soul to be the "form" of the body: i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body; spirit and matter, in man, are not two natures united, but rather [u]their union forms a single nature[/u].[/i]

If the union of body and soul forms a single nature, and that nature is our human nature, then what is our nature after death but before the resurrection, when our body and soul are not in union?

[Laudate_Dominum]

To understand that formulation of the body-soul relationship we ought to go to the sources, in this case Saint Thomas.
Regarding the soul Aquinas says, "to be separated from the body is not in accord with its nature". Well, maybe I'll just let the Angelic Doctor speak for himself:

Summa Theologiae, I, 76, I
[i]Is the intellectual principle united to the body as its form? [/i]
[quote][b]Objection 6.[/b] Further, whatever exists in a thing by reason of its nature exists in it always. But to be united to matter belongs to the form by reason of its nature; because form is the act of matter, not by an accidental quality, but by its own essence; otherwise matter and form would not make a thing substantially one, but only accidentally one. Therefore a form cannot be without its own proper matter. But the intellectual principle, since it is incorruptible, as was shown above (75, 6), remains separate from the body, after the dissolution of the body. Therefore the intellectual principle is not united to the body as its form.

On the contrary, According to the Philosopher, Metaph. viii (Did. vii 2), difference is derived from the form. But the difference which constitutes man is "rational," which is applied to man on account of his intellectual principle. Therefore the intellectual principle is the form of man.

I answer that, We must assert that the intellect which is the principle of intellectual operation is the form of the human body. For that whereby primarily anything acts is a form of the thing to which the act is to be attributed: for instance, that whereby a body is primarily healed is health, and that whereby the soul knows primarily is knowledge; hence health is a form of the body, and knowledge is a form of the soul. The reason is because nothing acts except so far as it is in act; wherefore a thing acts by that whereby it is in act. Now it is clear that the first thing by which the body lives is the soul. And as life appears through various operations in different degrees of living things, that whereby we primarily perform each of all these vital actions is the soul. For the soul is the primary principle of our nourishment, sensation, and local movement; and likewise of our understanding. Therefore this principle by which we primarily understand, whether it be called the intellect or the intellectual soul, is the form of the body. This is the demonstration used by Aristotle (De Anima ii, 2).

[b]Reply to Objection 6. [/b]To be united to the body belongs to the soul by reason of itself, as it belongs to a light body by reason of itself to be raised up. And as a light body remains light, when removed from its proper place, retaining meanwhile an aptitude and an inclination for its proper place; so the human soul retains its proper existence when separated from the body, having an aptitude and a natural inclination to be united to the body.
[/quote]

Summa Theologiae, I, 89, 1
[i]Can the soul separated from the body understand?[/i]
[quote]To solve this difficulty we must consider that as nothing acts except so far as it is actual, the mode of action in every agent follows from its mode of existence. Now the soul has one mode of being when in the body, and another when apart from it, its nature remaining always the same; but this does not mean that its union with the body is an accidental thing, for, on the contrary, such union belongs to its very nature, just as the nature of a light object is not changed, when it is in its proper place, which is natural to it, and outside its proper place, which is beside its nature. The soul, therefore, when united to the body, consistently with that mode of existence, has a mode of understanding, by turning to corporeal phantasms, which are in corporeal organs; but when it is separated from the body, it has a mode of understanding, by turning to simply intelligible objects, as is proper to other separate substances. Hence it is as natural for the soul to understand by turning to the phantasms as it is for it to be joined to the body; but to be separated from the body is not in accordance with its nature, and likewise to understand without turning to the phantasms is not natural to it; and hence it is united to the body in order that it may have an existence and an operation suitable to its nature. [/quote]

These are just snippets so you might want to read the whole thing on newadvent:
[url="http://www.newadvent.org/summa/1.htm"]http://www.newadvent.org/summa/1.htm[/url]
The most relevent questions are in the Prima Pars, numbers 75, 76, 80, 81, 88, 89, 90 and 91.

God bless