Thus, he learns to abstract flatness and roundness from the objects that express them, and to see shape as an abstraction a mental concept, he realizes at last, quite distinct from any specific object. Consciousness itself is the ultimate abstraction. Other abstractions, without it, would not exist. It is in the mind that flatness and roundness are recognized. It is our consciousness that convinces us of our existence.
Thoughts can only express consciousness, more or less as a tennis ball expresses roundness.
Descartes, searching for the key to existence, thought it necessary for him to have an object to study. Consciousness, however, is not an object, but the subject of anything seen. Consciousness is the universal constant, irreducible to any other abstraction. Happiness also is irreducible. It is expressed in countless ways.