Plato famously makes a distinction between the realm of the sensible (Becoming) and the intelligible (Being). The sensible realm is the one whose general determinable attribute is alteration. The intelligible realm is the one whose particular determinate attribute is an utter, mystical sameness. This latter realm is where Plato places Form (contentious view: not Forms), and this realm can only be accessed by a pure intellection that is supposed to transcend perceptions of the sensible alterations in the realm of Becoming.
Cue Descartes over a millennium later as he famously ruminates on the piece of wax...
“I now know that even bodies are not strictly perceived by the senses or the faculty of imagination but by the intellect alone, and that this perception derives not from their being touched or seen but from their being understood; and in view of this I know plainly that I can achieve an easier and more evident perception of my own mind than of anything else.” (Meditation Two)According to Descartes, even though the sensible qualities of the wax may change when I heat it, I still judge that the same wax remains through all the alterations. My distinct conception of the wax as an enduring material substance is not based on its changeable sensible qualities.
Rational reflection moves me to judge that the wax as a material body is “merely something extended, flexible and changeable.” (Meditation Two)
Since the wax is potentially infinitely flexible and changeable, my adequate conception of the wax could not be a function of my sensory imagination but rather of my rational understanding.
It’s obvious that Descartes is shopping for a general definition that is aimed to be “essence-tracking” with respect to not only the wax but of material substance as such. He’s looking for the Form.
This is his way of articulating the Socratic/Platonic search for answers to the “What is X?” question (e.g., What is dikaiosune? What is arête? etc.)