ioo Contradictory actions cannot co-exist. 



same instant. We can neither feel, perceive, nor 

 observe, one thing, whilst we are feeling, perceiving, 

 or observing, one of a contradictory nature ; nor can 

 we perform any two contradictory acts of comparison, 

 inference, imagination, or volition, simultaneously. 

 As also two mental actions are often not exactly 

 alike, or entirely harmonious, they must so far as they 

 are really contradictory, be mutually exclusive ; and 

 one of them must partly prevent the other, the 

 strongest one prevailing, and this general truth is 

 commonly though not explicitly, recognised in the 

 maxim, that to do anything well, we must do only 

 one thing at a time. In accordance with the univer- 

 sal truth, that contradictions cannot co-exist, it is well- 

 v known that one disease frequently expels another 

 from our frame, and the action of counter-irritants is 

 based upon the same principle. The fortitude of 

 martyrs may probably be explained by this power of 

 one set of ideas and feelings to exclude another, and 

 the facts of mental physiology afford plenty of other 

 examples. 



It is probably because we cannot simultaneously 

 perform two contradictory actions, that we cannot 

 contemplate consciousness, or think of an idea and at 

 the same time think of that act of thought. In ac- 

 cordance with this, even Newton, and other great 

 geniuses, have been unable to accurately describe the 

 mental processes by means of which they arrived at 

 their most difficult results. In consequence also of 

 this, we cannot define consciousness, and are often 



