Full Idea
Men many times fancy that they act from reason, when the reasons they attribute to themselves are nothing but excuses which unconscious instinct invents to satisfy the teasing 'whys' of the ego.
Gist of Idea
Men often answer inner 'whys' by treating unconscious instincts as if they were reasons
Source
Charles Sanders Peirce (Reasoning and the Logic of Things [1898], I)
Book Reference
Peirce,Charles Sanders: 'Reasoning and the Logic of Things', ed/tr. Ketner,K.L. [Harvard 1992], p.111
A Reaction
A strikely modern thought, supported by a lot of modern neuro-science and psychology. It is crucial to realise that we don't have to accept the best explanation we can think of.