Ética, Parte II, Proposición 26

If the human body is in no way affected by a given external body, then (II. vii.) neither is the idea of the human body, in other words, the human mind, affected in any way by the idea of the existence of the said external body, nor does it in any manner perceive its existence. But, in so far as the human body is affected in any way by a given external body, thus far (II. xvi. and Coroll.) it perceives that external body. Q.E.D.

In so far as the human mind imagines an external body, it has not an adequate knowledge thereof.

When the human mind regards external bodies through the ideas of the modifications of its own body, we say that it imagines (see II. xvii. note); now the mind can only imagine external bodies as actually existing. Therefore (by II. xxv.), in so far as the mind imagines external bodies, it has not an adequate knowledge of them. Q.E.D.