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 ( Lk. 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 (Q[88], A[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 (A[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 (Q[87], A[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.
Whether the separated soul knows all natural things?
Objection 1: It would seem that the separated soul knows all natural things. For the types of all natural things exist in separate substances. Therefore, as separated souls know separate substances, they also know all natural things.
Objection 2: Further, whoever understands the greater intelligible, will be able much more to understand the lesser intelligible. But the separated soul understands immaterial substances, which are in the highest degree of intelligibility. Therefore much more can it understand all natural things which are in a lower degree of intelligibility.
On the contrary, The devils have greater natural knowledge than the separated soul; yet they do not know all natural things, but have to learn many things by long experience, as Isidore says (De Summo Bono i). Therefore neither can the separated soul know all natural things.
I answer that, As stated above (A[1]), the separated soul, like the angels, understands by means of species, received from the influence of the Divine light. Nevertheless, as the soul by nature is inferior to an angel, to whom this kind of knowledge is natural, the soul apart from the body through such species does not receive perfect knowledge, but only a general and confused kind of knowledge. Separated souls, therefore, have the same relation through such species to imperfect and confused knowledge of natural things as the angels have to the perfect knowledge thereof. Now angels through such species know all natural things perfectly; because all that God has produced in the respective natures of natural things has been produced by Him in the angelic intelligence, as Augustine says (Gen. ad lit. ii, 8). Hence it follows that separated souls know all natural things not with a certain and proper knowledge, but in a general and confused manner.
Reply to Objection 1: Even an angel does not understand all natural things through his substance, but through certain species, as stated above (Q[87], A[1]). So it does not follow that the soul knows all natural things because it knows separate substances after a fashion.
Reply to Objection 2: As the soul separated from the body does not perfectly understand separate substances, so neither does it know all natural things perfectly; but it knows them confusedly, as above explained in this article.
Reply to Objection 3: Isidore speaks of the knowledge of the future which neither angels, nor demons, nor separated souls, know except so far as future things pre-exist in their causes or are known by Divine revelation. But we are here treating of the knowledge of natural things.
Reply to Objection 4: Knowledge acquired here by study is proper and perfect; the knowledge of which we speak is confused. Hence it does not follow that to study in order to learn is useless.
Whether the separated soul knows singulars?
Objection 1: It would seem that the separated soul does not know singulars. For no cognitive power besides the intellect remains in the separated soul, as is clear from what has been said above (Q[77], A[8]). But the intellect cannot know singulars, as we have shown (Q[86], A[1]). Therefore the separated soul cannot know singulars.
Objection 2: Further, the knowledge of the singular is more determinate than knowledge of the universal. But the separated soul has no determinate knowledge of the species of natural things, therefore much less can it know singulars.
Objection 3: Further, if it knew the singulars, yet not by sense, for the same reason it would know all singulars. But it does not know all singulars. Therefore it knows none.
On the contrary, The rich man in hell said: "I have five brethren" ( Lk. 16:28 ).
I answer that, Separated souls know some singulars, but not all, not even all present singulars. To understand this, we must consider that there is a twofold way of knowing things, one by means of abstraction from phantasms, and in this way singulars cannot be directly known by the intellect, but only indirectly, as stated above (Q[86], A[1]). The other way of understanding is by the infusion of species by God, and in that way it is possible for the intellect to know singulars. For as God knows all things, universal and singular, by His Essence, as the cause of universal and individual principles (Q[14], A[2]), so likewise separate substances can know singulars by species which are a kind of participated similitude of the Divine Essence. There is a difference, however, between angels and separated souls in the fact that through these species the angels have a perfect and proper knowledge of things; whereas separated have only a confused knowledge. Hence the angels, by reason of their perfect intellect, through these species, know not only the specific natures of things, but also the singulars contained in those species; whereas separated souls by these species know only those singulars to which they are determined by former knowledge in this life, or by some affection, or by natural aptitude, or by the disposition of the Divine order; because whatever is received into anything is conditioned according to the mode of the recipient.
Reply to Objection 1: The intellect does not know the singular by way of abstraction; neither does the separated soul know it thus; but as explained above.
Reply to Objection 2: The knowledge of the separated soul is confined to those species or individuals to which the soul has some kind of determinate relation, as we have said.
Reply to Objection 3: The separated soul has not the same relation to all singulars, but one relation to some, and another to others. Therefore there is not the same reason why it should know all singulars.
Whether the habit of knowledge here acquired remains in the separated soul?
Objection 1: It would seem that the habit of knowledge acquired in this life does not remain in the soul separated from the body: for the Apostle says: "Knowledge shall be destroyed" ( 1 Cor. 13:8 ).