3. Of Relation: Substance and accident, cause and effect, action and
reaction.
4. Of Modality: Possibility--impossibility, existence--non-existence,
necessity--contingence.
These, then, are the fundamental, primary, or native conceptions of the
understanding, which flow from, or constitute the mechanism of, its
nature; are inseparable from its activity; and are hence, for human
thought, universal and necessary, or _a priori_. These categories are
"pure" conceptions of the understanding, inasmuch as they are
independent of all that is contingent in sense.