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 (Article 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 (Question 87, Article 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.
Source: Summa Theologica (New Advent)