In 1923 he was elected a Fellow of the university, allowing him to attend Oriel College, Oxford, where he gained a DPhil in 1928 for a dissertation on "The History and Value of the Distinction between Intellect and Intuition".

[3] After the retirement of W. Jenkyn Jones in 1932, Aaron was appointed to the chair of philosophy at University College of Wales, Aberystwyth where he settled, initially on the nearby hill of Bryn Hir and later at Garth Celyn.

The interest was sparked by his discovery of unexamined information in the Lovelace Collection: notes and drafts left by John Locke to his cousin Peter King.

There he found letters, notebooks, catalogues, and most pertinently, an early draft of Locke's "An Essay Concerning Human Understanding", hitherto presumed missing.

In 1971, he published a third edition of his Locke biography and the book Knowing and the Function of Reason, which includes broad discussion of the laws of non-contradiction, excluded middle and identity, of the use of language in speech and thought, and of substance and causality.

[1] After retiring in 1969, he taught for a semester at Carlton College, Minnesota, before returning to Wales, where he helped to write articles for the 1974 edition of the Encyclopædia Britannica.