HKB
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Often appears with: reason and the intellect · nominal essence · language, naming, and meaning · essence and species · doubt, skepticism, and certainty · certainty
Usually: reflection (1) · crisis of belief (1) · detachment (1)
“For man or gold, taken in this sense, and used for species of things constituted by real essences, different from the complex idea in the mind of the speaker, stand for we know not what; and the extent of these species, with such boundaries, are so unknown and undetermined.” John Locke, An Essay Concerning Humane Understanding, Volume 2 · observation
He argues that certainty about general propositions requires knowing the precise bounds of each species, which is easy for simple ideas but uncertain for substances with supposed real essences we do not know.
precise bounds and extent of the species its terms stand for, it is necessary we should know the essence of each species, which is that which constitutes and bounds it. This, in all simple ideas and modes, is not hard to do. For in these the real and nominal essence being the same, or, which is all one, the abstract idea which the general term stands for being the sole essence and boundary that is or can be supposed…