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Full Idea
We distinguish the grasp of a thought, which is 'thinking', from the acknowledgement of the truth of a thought, which is the act of 'judgement', from the manifestation of this judgement, which is an 'assertion'.
Gist of Idea
We grasp thoughts (thinking), decide they are true (judgement), and manifest the judgement (assertion)
Source
Gottlob Frege (The Thought: a Logical Enquiry [1918], p.329 (62))
Book Ref
Frege,Gottlob: 'The Frege Reader', ed/tr. Beaney,Michael [Blackwell 1997], p.329