Charles Darwin once wrote:           

But then with me the horrid doubt always arises 

whether the convictions of man's mind,

which has been developed from the mind of the lower animals,

are of any value or at all trustworthy.

 Would any one trust in the convictions of a monkey's mind,    

if there are any convictions in such a mind?

Reference: The Complete Work of Charles Darwin Online. Page 316.

http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F1452.1&pageseq=334

Reference: Charles Darwin's Letter to William Graham, July 3,1881. 

http://www.darwinproject.ac.uk/darwinletters/calendar/entry-13230.html