Having introduced Leibniz's concepts of unconscious perceptions and appetites, the question arises whether there is contemporary empirical evidence for their existence. In Leibniz's own time the assumption that there are unnoticeable and therefore unconscious perceptions was an uncommon one. As the brief references to Hobbes, Locke, and Descartes have already indicated in the previous chapter, this assumption was either simply not entertained or even strongly opposed to by most other early modern philosophers, both for metaphysical reasons and because of a lack of (direct) empirical evidence in favor of their existence.