You must consider, when reading this treatise, that mental perception, because connected with matter, is subject to conditions similar to those to which physical perception is subject.

You must accept the truth from whatever source it comes.

Do not consider it proof just because it is written in books, for a liar who will deceive with his tongue will not hesitate to do the same with his pen.

You must also give mental and physical fitness priority.

The same is the case with those opinions of man to which he has been accustomed from his youth; he likes them, defends them, and shuns the opposite views.

Further, there are things of which the mind understands one part, but remains ignorant of the other; and when man is able to comprehend certain things, it does not follow that he must be able to comprehend everything.