  Thus as Descartes held, there are three kinds of ideas (adventitious, fictitious, and innate), which are distinguished by their differing sources as well as the way in which they are inbred in the mind. Adventitious ideas proceed from the senses, fictitious ideas proceed from the imagination and the intellect, and innate ideas proceed from thought itself, which acts as their ground or original. Nonetheless, all ideas, regardless of their source or origin, depend on the mind in some essential way for their form. But this gives rise to the problem of explaining how ideas can be said to represent, if they in no way resemble their objects. This problem is especially acute for Cartesians who held that there is a modal difference between what is found at the level of sensory perception and intellection, such that impressions cannot contain any of the properties found at the level of ideas. What arrives at our faculty of thinking from the senses is not ideas such as we form them in our thought, as the scholastic empiricists held, but rather only various particular motions emitted by external objects. (1694, p. 328)