Scottish metaphysician distinguished for his theory of agnoiology, or theory of ignorance.

Educated at Edinburgh and Oxford, Ferrier qualified as a barrister in 1832, but he came under the influence of the Scottish philosopher Sir William Hamilton (who may have inspired his visit to Heidelberg in 1834 to study German idealist philosophy) and was appointed professor of civil history at Edinburgh University (1842) and then of moral philosophy and political economy at the University of St. Andrews (1845).

Ferrier's Hegelian epistemology (a word that he introduced into English) and ontology are based on the concept of the unity of the act of knowledge, which combines the knowing subject and the object known. In his view, the mind cannot apprehend anything except in conjunction with an apprehension of itself, and the distinction of subject and object is a source of error. Only minds in synthesis with what they know can be said to exist. Thus, a mind cannot be “ignorant” of what is allegedly unknowable (as the Kantian “thing-in-itself” was said to be), since ignorance must refer to what is still knowable though not actually known. Ferrier's major work was Institutes of Metaphysic, the Theory of Knowing and Being (1854).