“Thus what I thought I had seen with my eyes, I actually grasped solely with the faculty of judgment, which is in my mind.”

As one means of demonstrating the limitations of the senses, Descartes puts forth what is known as the Wax Argument in Discourse on Method and Meditations on First Philosophy .  He considers a piece of wax and notes that his senses inform him that it has certain properties, such as shape, texture, size, color, and smell.  However, when Descartes brings the piece of wax into contact with flame, all of these properties change, for the heat from the flame melts the wax.  Yet it seems that this entity is still the same wax, in spite of its different appearance—it is still a piece of wax, even though the data from Descartes' senses inform him that all of its characteristics are different than they appeared originally.  Therefore, Descartes concludes that in order to properly grasp the true nature of the wax, one cannot rely on the senses, but must instead use one’s rational faculty.  Time and memory play an important role in judgment; it is in part due to the memory of the original appearance of the wax at a time prior to that when it melts that one is able to determine that it is in fact the same piece of wax.