HUME'S TOUCHSTONE (original version)
 

"This doctrine ["that beasts are endow'd with thought and reason as well as men"] is as useful as it is obvious, and furnishes us with a kind of touchstone by which we may try every system in this species of philosophy [theories of mind]. 'Tis from the resemblance of the external actions of animals to those we ourselves perform that we judge their internal likewise to resemble ours; and the same principle of reasoning carry'd one step farther will make us conclude that since our internal actions resemble each other, the causes from which they are deriv'd must also be resembling. When any hypothesis, therefore, is advanc'd to explain a mental operation which is common to men and beasts, we must apply the same hypothesis to both; and as every true hypothesis will abide this trial, so I may venture to affirm that no false one will ever be able to endure it." (David Hume, A Treatise of Human Nature, Book I, Part III, Sec. 16.)
 

HUME'S TOUCHSTONE (revised version)
 

"All our reasonings concerning matter of fact are founded on a species of ANALOGY which leads us to expect from any cause the same events which we have observed to result from similar causes. Where the causes are entirely similar, the analogy is perfect and the inference drawn from it is regarded as certain and conclusive. ...But where the objects have not so exact a similarity, the analogy is less perfect and the inference is less conclusive, though still it has some force in proportion to the degree of similarity and resemblance. ... These analogical observations may be carried farther, even to this science of which we are now treating; and any theory by which we explain the operations of the understanding or the origin and connexion of the passions in man will acquire additional authority if we find that the same theory is requisite to explain the same phenomena in all other animals."

(David Hume, An Enquiry concerning Human Understanding, Sec. IX.)