It's usually best to start with a keyword search, but subject headings applied to relevant books can help to refine your results. Open the item record of a book that interests you. Scroll down to the clickable subject headings, and run another search to explore similar books.
You can also browse the headings associated with a particular topic as a way of getting started with your research.
Titles on ancient philosopy are cataloged with the broad subject heading Philosophy, Ancient.
Some examples are: