Objection 1. It would seem that knowledge is not the proper matter of studiousness. For a person is said to be studious because he applies study to certain things. Now a man ought to apply study to every matter, in order to do aright what has to be done. Therefore seemingly knowledge is not the special matter of studiousness.
Objection 2. Further, studiousness is opposed to curiosity. Now curiosity, which is derived from "cura" [care], may also refer to elegance of apparel and other such things, which regard the body; wherefore the Apostle says (Romans 13:14): "Make not provision [curam] for the flesh in its concupiscences."
Objection 3. Further it is written (Jeremiah 6:13): "From the least of them even to the greatest, all study [Douay: 'are given to'] covetousness." Now covetousness is not properly about knowledge, but rather about the possession of wealth, as stated above (Question 118, Article 2). Therefore studiousness, which is derived from "study," is not properly about knowledge.
On the contrary, It is written (Proverbs 27:11): "Study wisdom, my son, and make my heart joyful, that thou mayest give an answer to him that reproacheth." Now study, which is commended as a virtue, is the same as that to which the Law urges. Therefore studiousness is properly about "knowledge."
I answer that, Properly speaking, study denotes keen application of the mind to something. Now the mind is not applied to a thing except by knowing that thing. Wherefore the mind's application to knowledge precedes its application to those things to which man is directed by his knowledge. Hence study regards knowledge in the first place, and as a result it regards any other things the working of which requires to be directed by knowledge. Now the virtues lay claim to that matter about which they are first and foremost; thus fortitude is concerned about dangers of death, and temperance about pleasures of touch. Therefore studiousness is properly ascribed to knowledge.
Reply to Objection 1. Nothing can be done aright as regards other matters, except in so far as is previously directed by the knowing reason. Hence studiousness, to whatever matter it be applied, has a prior regard for knowledge.
Reply to Objection 2. Man's mind is drawn, on account of his affections, towards the things for which he has an affection, according to Matthew 6:21, "Where thy treasure is, there is thy heart also." And since man has special affection for those things which foster the flesh, it follows that man's thoughts are concerned about things that foster his flesh, so that man seeks to know how he may best sustain his body. Accordingly curiosity is accounted to be about things pertaining to the body by reason of things pertaining to knowledge.
Reply to Objection 3. Covetousness craves the acquisition of gain, and for this it is very necessary to be skilled in earthly things. Accordingly studiousness is ascribed to things pertaining to covetousness.
Source: Summa Theologica (New Advent)