We are not interested in examining how far in our assertion of the pleasure-principle we have approached to or adopted any given philosophical system historically established. Our approach to such speculative hypotheses is by way of our endeavour to describe and account for the facts falling within our daily sphere of observation. Priority and originality are not among the aims which psycho-analysis sets itself, and the impressions on which the statement of this principle is founded are of so unmistakable a kind that it is scarcely possible to overlook them. On the other hand, we should willingly acknowledge our indebtedness to any philosophical or psychological theory that could tell us the meaning of these feelings of pleasure and ‘pain’ which affect us so powerfully. Unfortunately no theory of any value is forthcoming. It is the obscurest and least penetrable region of psychic life and, while it is impossible for us to avoid touching on it, the most elastic hypothesis will be, to my mind, the best. We have decided to consider pleasure and ‘pain’ in relation to the quantity of excitation present in the psychic life—and not confined in any way—along such lines that ‘pain’ corresponds with an increase and pleasure with a decrease in this quantity. We do not thereby commit ourselves to a simple relationship between the strength of the feelings and the changes corresponding with them, least of all, judging from psycho-physiological experiences, to any view of a direct proportion existing between them; probably the amount of diminution or increase in a given time is the decisive factor for feeling. Possibly there is room here for experimental work, but it is inadvisable for us analysts to go further into these problems until we can be guided by quite definite observations. We cannot however profess the like indifference when we find that an investigator of such penetration as G. Th. Fechner has advocated a conception of pleasure and ‘pain’ which in essentials coincides with that forced upon us by psycho-analytic work. Fechner’s pronouncement is to be found in his short work ‘Einige Ideen zur Schöpfungs- und Entwicklungsgeschichte der Organismen’, 1873 (Section XI, Note p. 94) and reads as follows: ‘In so far as conscious impulses always bear a relation to pleasure or “pain”, pleasure or “pain” may be thought of in psycho-physical relationship to conditions of stability and instability, and upon this may be based the hypothesis I intend to develop elsewhere, viz.: that every psycho-physical movement rising above the threshold of consciousness is charged with pleasure in proportion as it approximates—beyond a certain limit—to complete equilibrium, and with “pain” in proportion as it departs from it beyond a certain limit; while between the two limits which may be described as the qualitative thresholds of “pain” or pleasure, there is a certain area of aesthetic indifference.’