Theory deriving from Johannes Duns Scotus (c. 1266-1308), with roots in Aristotle, that as well as ordinary general properties there are special properties (haec-ceitie or thisnesses) necessarily associated each with just one individual.
Traditional Aristotelianism individuated objects by their matter, which as such (abstracted from all form) was unknowable.
Properties, however, count as form rather than matter, and so Socrates and Plato could now - at least in principle - be distinguished by reason, not just by the senses. Recently, 'anti-haecceitism' has been used for the rejection either of primitive (that is unanalyzable) thisnesses or of primitive transworld identity.
Also see: counterpart theory
A B Wolter, The Philosophical Theology of John Duns Scotus, M McC Adams, ed. (1990), ch. 4