INTRODUCTION
Looking over the history of “Denkpsychologie” I found it strange that the problem of the process of doubt has received but slight attention at the hands of research workers. Being interested in the morphological aspect of the consciousness of doubt, I made an attempt, in my present investigation, to describe and analyse the consciousness of doubt.
The present investigation of the process of doubt was carried on in the Psychogical Laboratory of Hosei University during last winter, under the direction of Prof. Kido and Prof. Mitsui. The experiments were divided into two parts. In the first part I observed the following three simpler mohtal processes:
1. The discrimination of two tactual impressions given by aesthesiometer on the velar side of the fore arm.
2. The comparison of two distances perceived by a subject as he bends twice his forearm; the standard stimulus being 30cm and the comparison stimuli of various lengths.
3. The judgment of a tone whether the middle tone is heard or not, three tones being given, higher, midde and lower. In this experiment a monocord was employed to give tones.
To each of these experiments I applied both the method of minimal changes and that of right-and-wrong cases.
In the second part of my experiments I made an observation of much more complex mental processes; that is, the arrange- ment and classification of various cards, all of which are different in color, form, mark or in their combinations.
RESULTS
Although the magnitudes of stimuli necessary for the occur- rences of the consciousness of doubt were various with different subjects, still I was able to find a regular relation between the magnitude of stimuli and the occurrence of the consciousness of doubt. In the discrimination of two tactual impressions upon the velar side of the forearm, all the subjects were observed to fall in doubt when they came into certain range, that is, between a point at which they began to feel indistinctly the given impres- sion as a single one and a point at which they felt it clearly as two separate ones. In the comparison of two distances, all the subjects were observed to fall in doubt when the comparison stimuli were equal to, or slightly longer or shorter than, the standard stimulus. In the judgment of a tone whether it was heard as the middle tone or not, all the subjects were observed to fall in doubt when the tone required to be judged was equal to, or slightly higher or lower than, the objectively middle pitched tone.
It may be noticed here that these regular occurrences of the conscioness of doubt do not mean there is no possibility of other irregular occurrences of it under various conditions. Under what conditions do the subjects come necessarily to be doubtful ? What kinds of doubt can there be found ? In order to throw a light on these questions, let me look into all the records of the introspection of the subjects.
The occurrences of the consciousness of doubt seem to depend on the following conditions:-
1. The inadequateness of impressions,-
a. Intensity and pain-sensation.
In the discrimination of two tactual impressions Sub. T. introspected, “At first I thought I had perceived only a single impression. But as its intensity was very weak I can not tell it precisely.”
In the judgment of a tone whether it was heard as the middle tone or not, Sub. Y. introspected, “I thought it was a middle tone as I had expected to be. But as its intensity was very strong, I can not tell it precisely.”
b. Some accidental sensations.
In the comparison of t we distances Sub. T. introspected, “At the moment the arm-movement came to an end I heard a noise at the top of the arm-holder. By the aid of the noise, I could immediately visualize the distance.

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