Objection 1. It would seem that the soul separated from the body can understand nothing at all. For the Philosopher says (De Anima i, 4) that "the understanding is corrupted together with its interior principle." But by death all human interior principles are corrupted. Therefore also the intellect itself is corrupted.
Objection 2. Further, the human soul is hindered from understanding when the senses are tied, and by a distracted imagination, as explained above (84, 7,8). But death destroys the senses and imagination, as we have shown above (Question 77, Article 8). Therefore after death the soul understands nothing.
Objection 3. Further, if the separated soul can understand, this must be by means of some species. But it does not understand by means of innate species, because it has none such; being at first "like a tablet on which nothing is written": nor does it understand by species abstracted from things, for it does not then possess organs of sense and imagination which are necessary for the abstraction of species: nor does it understand by means of species, formerly abstracted and retained in the soul; for if that were so, a child's soul would have no means of understanding at all: nor does it understand by means of intelligible species divinely infused, for such knowledge would not be natural, such as we treat of now, but the effect of grace. Therefore the soul apart from the body understands nothing.
On the contrary, The Philosopher says (De Anima i, 1), "If the soul had no proper operation, it could not be separated from the body." But the soul is separated from the body; therefore it has a proper operation and above all, that which consists in intelligence. Therefore the soul can understand when it is apart from the body.
I answer that, The difficulty in solving this question arises from the fact that the soul united to the body can understand only by turning to the phantasms, as experience shows. Did this not proceed from the soul's very nature, but accidentally through its being bound up with the body, as the Platonists said, the difficulty would vanish; for in that case when the body was once removed, the soul would at once return to its own nature, and would understand intelligible things simply, without turning to the phantasms, as is exemplified in the case of other separate substances. In that case, however, the union of soul and body would not be for the soul's good, for evidently it would understand worse in the body than out of it; but for the good of the body, which would be unreasonable, since matter exists on account of the form, and not the form for the sake of matter. But if we admit that the nature of the soul requires it to understand by turning to the phantasms, it will seem, since death does not change its nature, that it can then naturally understand nothing; as the phantasms are wanting to which it may turn.
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. But here again a difficulty arises. For since nature is always ordered to what is best, and since it is better to understand by turning to simply intelligible objects than by turning to the phantasms; God should have ordered the soul's nature so that the nobler way of understanding would have been natural to it, and it would not have needed the body for that purpose.
In order to resolve this difficulty we must consider that while it is true that it is nobler in itself to understand by turning to something higher than to understand by turning to phantasms, nevertheless such a mode of understanding was not so perfect as regards what was possible to the soul. This will appear if we consider that every intellectual substance possesses intellective power by the influence of the Divine light, which is one and simple in its first principle, and the farther off intellectual creatures are from the first principle so much the more is the light divided and diversified, as is the case with lines radiating from the centre of a circle. Hence it is that God by His one Essence understands all things; while the superior intellectual substances understand by means of a number of species, which nevertheless are fewer and more universal and bestow a deeper comprehension of things, because of the efficaciousness of the intellectual power of such natures: whereas the inferior intellectual natures possess a greater number of species, which are less universal, and bestow a lower degree of comprehension, in proportion as they recede from the intellectual power of the higher natures. If, therefore, the inferior substances received species in the same degree of universality as the superior substances, since they are not so strong in understanding, the knowledge which they would derive through them would be imperfect, and of a general and confused nature. We can see this to a certain extent in man, for those who are of weaker intellect fail to acquire perfect knowledge through the universal conceptions of those who have a better understanding, unless things are explained to them singly and in detail. Now it is clear that in the natural order human souls hold the lowest place among intellectual substances. But the perfection of the universe required various grades of being. If, therefore, God had willed souls to understand in the same way as separate substances, it would follow that human knowledge, so far from being perfect, would be confused and general. Therefore to make it possible for human souls to possess perfect and proper knowledge, they were so made that their nature required them to be joined to bodies, and thus to receive the proper and adequate knowledge of sensible things from the sensible things themselves; thus we see in the case of uneducated men that they have to be taught by sensible examples.
It is clear then that it was for the soul's good that it was united to a body, and that it understands by turning to the phantasms. Nevertheless it is possible for it to exist apart from the body, and also to understand in another way.
Reply to Objection 1. The Philosopher's words carefully examined will show that he said this on the previous supposition that understanding is a movement of body and soul as united, just as sensation is, for he had not as yet explained the difference between intellect and sense. We may also say that he is referring to the way of understanding by turning to phantasms. This is also the meaning of the second objection.
Reply to Objection 3. The separated soul does not understand by way of innate species, nor by species abstracted then, nor only by species retained, and this the objection proves; but the soul in that state understands by means of participated species arising from the influence of the Divine light, shared by the soul as by other separate substances; though in a lesser degree. Hence as soon as it ceases to act by turning to corporeal (phantasms), the soul turns at once to the superior things; nor is this way of knowledge unnatural, for God is the author of the influx of both of the light of grace and of the light of nature.
Source: Summa Theologica (New Advent)