An individual idea cannot be the subject of a judgment, because it is not an abstraction, it is not something thought, but something perceived.
 Arthur Schopenhauer, The World as Will and Representation (1819). copy citation

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Author Arthur Schopenhauer
Source The World as Will and Representation
Topic judgment abstraction
Date 1819
Language English
Reference
Note Translated by R. B. Haldane and J. Kemp
Weblink http://www.gutenberg.org/files/40097/40097-h/40097-h.html

Context

“operating with judgments, this makes no difference, simply because between singular and universal conceptions there is no logical difference. “Immanuel Kant” signifies logically, “all Immanuel Kant.” Accordingly the quantity of judgments is really only of two kinds—universal and particular. An individual idea cannot be the subject of a judgment, because it is not an abstraction, it is not something thought, but something perceived. Every conception, on the other hand, is essentially universal, and every judgment must have a conception as its subject. The difference between particular judgments (propositiones particulares) and universal judgments often depends merely on the external and contingent circumstance that the language has no word to express by itself the part that is here to be separated from the general conception which forms the subject of such a judgment.” source