In addition to faculties of perception and memory, Reid posited a moral faculty resulting in conceptions of justice or injustice that may differ, depending on different people’s conceptions of the same action. He also posited active powers, leading to action, according to principles of action. When Reid spoke of “powers” in this way, he seemed to mean capabilities in the mind. The principles of action were animal principles (such as appetites and physical desires) and rational principles that include understanding and will.

Reid believed that we are able to will something because we have a conception of the action and we will to do that thing. Concerning freedom, Reid thought it was not sufficient, as David Hume (1711–1776) had claimed, that we act according to our will, but that we must also have the power to choose what to will. This is because willing is instrumental to the goal of an action, and without the power over means there is no power over the end. Your free actions are those caused by you and you know that you are their cause because your conviction of your freedom arises from your faculties, as a first principle.