In 1926 Aaron was appointed a lecturer in the Department of Philosophy at the
University College of Swansea. Aaron produced several more books and articles, including a book in
Welsh on the history of philosophy,
Hanes athroniaeth—o Descartes i Hegel in 1932. Other notable publications of Aaron's include "Two Senses of the Word Universal" (1939) and "Our Knowledge of Universals", a study read to the
British Academy in 1945 and published in its
Proceedings and as a separate monograph. Aaron's fascination with the idea of a
universal culminated in
The Theory of Universals (1952). Here, he attacks the notion of universals as
Platonic forms, but is as critical of
Aristotelian realism on
essences as he is of
nominalism and
conceptualism as theories of universals. In 1952–1953, Aaron was a visiting professor at
Yale University. In 1956, he was able to study the third draft of Locke's
An Essay Concerning Human Understanding at the
Pierpont Morgan Library, which led to a substantial addition to the second edition of
John Locke, published in 1955. He became a
Fellow of the British Academy (FBA) and president of the
Mind Association in the same year. In 1956, the Joint Session of the
Aristotelian Society and Mind Association (publisher of the journal
Mind) was held in
Aberystwyth, and Aaron gave the inaugural lecture. In 1957 he was elected President of the Aristotelian Society. ==Selected works==