Full Idea
If the human body is affected in a manner which involves the nature of any external body, the human mind will regard the said external body as actually existing.
Gist of Idea
If the body is affected by an external object, the mind can't help believing that the object exists
Source
Baruch de Spinoza (The Ethics [1675], II Pr 17)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.98
A Reaction
This is like one of Hume's 'natural beliefs', and seems to me a powerful idea. One of the basic questions of epistemology is, apart from the question 'which beliefs can I justify?', also 'which beliefs can I never abandon?' Skip the scepticism?