TRIAL AND ERROR REACTIONS IN MAMMALS 49 



to pressure until he tries it. Of these three inferentially possible 

 doors of exit, two are actually locked, and one is unlocked. 

 We shall speak, therefore, of " locked possible doors " and an 



unlocked possible door. ' ' 



The intention of our investigation requires a division of all 

 the reactions of the subjects into two groups, only one of which 

 can enter into the tables. These groups are: 



(i) Unclassified reactions. This group includes all reactions 

 which led to the discovery of the unlocked door before all three 

 possible doors were tried, and which did not include more than 

 a single effort to open any given door during the trial. I have 

 rejected from the tables as " unclassified " all reactions which 

 met the above conditions, even though such a reaction in- 

 cluded an effort to open the impossible door. This is justified, 

 I believe, by the fact that none of the animal subjects seemed 

 to have a consistent awareness of the impossible door as such. 

 The description of classified reactions will disclose an additional 

 reason for this exclusion. 



(2) Classified reactions. To fall within this group a reaction 

 must meet one of the following* conditions : (a) Efforts to open 

 each of the three possible doors; (b) more than one separately 

 continuous effort to open a given door during the trial. For 

 example, if during a present trial, door 3 were the unlocked 

 door of the immediately preceding trial and door 2 the unlocked 

 door of the present trial, a reaction which could be tabulated 

 according to any of the following formulae would fall within 

 the classified group: Efforts to open exit doors 4, i, 2 in the 

 order given; or exit doors i, 4, i, 2 ; or exit doors 4, 4, i, 2, etc. 



The classified modes of searching for the unlocked door were 

 found to belong to five objectively different general types, which 

 may be described as follows: 



Type A. All three possible doors tried, once each; no effort 

 made to open the impossible door. 



This is the most adequate possible type of classified reactions. 



Type B. All four exit doors are tried, once each, and in an 

 irregular order. 



Type C. This reaction can occur only when the door to the 

 extreme right (Door 4), or the one to the extreme left (Door i) 

 is the unlocked door. It involves trying each of the four exit 

 doors once, and in order from left to right or from right to left, 



