Few men can think long without running into a confusion of ideas, and mistaking one for another
 David Hume, An Enquiry Concerning Human Understanding (1748). copy citation

Context

“Where there is a complication of causes to produce any effect, one mind may be much larger than another, and better able to comprehend the whole system of objects, and to infer justly their consequences. 3. One man is able to carry on a chain of consequences to a greater length than another. 4. Few men can think long without running into a confusion of ideas, and mistaking one for another; and there are various degrees of this infirmity. 5. The circumstance, on which the effect depends, is frequently involved in other circumstances, which are foreign and extrinsic. The separation of it often requires great attention, accuracy, and subtilty.” source