  This aspect was brought to the fore by Thomas Aquinas in what has come to be known as ‘the objective turn’. In his commentary on Boethius’ De trinitate, Aquinas advances the thesis that “to each [cognitive] power, its proper object is first known” (Super Boethium De trinitate, q. 1, art. 3). Since what specifies a cognitive power is its proper object – i.e. the formal aspect under which something is represented by that power –, the identification of the first known with the proper object of the intellect presents a significant claim. What is first in the order of conceptual cognition is most common and self-evident not by accident, for it indicates the formal aspect of intelligibility, which is included in all knowledge and about which the intellect cannot be uncertain. As such, it indicates the horizon of cognition. This promotion of a most common element of knowledge to the very horizon of knowledge, which was broadly acknowledged in the medieval debate on the first known, turned the medieval debate into a critique of knowledge.