Objection 1. It would seem that the separated soul does not understand separate substances. For the soul is more perfect when joined to the body than when existing apart from it, being an essential part of human nature; and every part of a whole is more perfect when it exists in that whole. But the soul in the body does not understand separate substances as shown above (Question 88, Article 1). Therefore much less is it able to do so when apart from the body.
Objection 2. Further, whatever is known is known either by its presence or by its species. But separate substances cannot be known to the soul by their presence, for God alone can enter into the soul; nor by means of species abstracted by the soul from an angel, for an angel is more simple than a soul. Therefore the separated soul cannot at all understand separate substances.
Objection 3. Further, some philosophers said that the ultimate happiness of man consists in the knowledge of separate substances. If, therefore, the separated soul can understand separate substances, its happiness would be secured by its separation alone; which cannot be reasonably be said.
On the contrary, Souls apart from the body know other separated souls; as we see in the case of the rich man in hell, who saw Lazarus and Abraham (Luke 16:23). Therefore separated souls see the devils and the angels.
I answer that, Augustine says (De Trin. ix, 3), "our mind acquires the knowledge of incorporeal things by itself"--i.e. by knowing itself (88, 1, ad 1). Therefore from the knowledge which the separated soul has of itself, we can judge how it knows other separate things. Now it was said above (Article 1), that as long as it is united to the body the soul understands by turning to phantasms, and therefore it does not understand itself save through becoming actually intelligent by means of ideas abstracted from phantasms; for thus it understands itself through its own act, as shown above (Question 87, Article 1). When, however, it is separated from the body, it understands no longer by turning to phantasms, but by turning to simply intelligible objects; hence in that state it understands itself through itself. Now, every separate substance "understands what is above itself and what is below itself, according to the mode of its substance" (De Causis viii): for a thing is understood according as it is in the one who understands; while one thing is in another according to the nature of that in which it is. And the mode of existence of a separated soul is inferior to that of an angel, but is the same as that of other separated souls. Therefore the soul apart from the body has perfect knowledge of other separated souls, but it has an imperfect and defective knowledge of the angels so far as its natural knowledge is concerned. But the knowledge of glory is otherwise.
Reply to Objection 1. The separated soul is, indeed, less perfect considering its nature in which it communicates with the nature of the body: but it has a greater freedom of intelligence, since the weight and care of the body is a clog upon the clearness of its intelligence in the present life.
Reply to Objection 2. The separated soul understands the angels by means of divinely impressed ideas; which, however, fail to give perfect knowledge of them, forasmuch as the nature of the soul is inferior to that of an angel.
Reply to Objection 3. Man's ultimate happiness consists not in the knowledge of any separate substances; but in the knowledge of God, Who is seen only by grace. The knowledge of other separate substances if perfectly understood gives great happiness--not final and ultimate happiness. But the separated soul does not understand them perfectly, as was shown above in this article.
Source: Summa Theologica (New Advent)