Full Idea
A thing (an object of our thought) is completely determined by all that can be affirmed or thought concerning it.
Gist of Idea
A thing is completely determined by all that can be thought concerning it
Source
Richard Dedekind (Nature and Meaning of Numbers [1888], I.1)
Book Reference
Dedekind,Richard: 'Essays on the Theory of Numbers' [Dover 1963], p.44
A Reaction
How could you justify this as an observation? Why can't there be unthinkable things (even by God)? Presumably Dedekind is offering a stipulative definition, but we may then be confusing epistemology with ontology.