In popular language the term pessimist is applied to persons who habitually take a melancholy view of life, to whom painful experiences appeal with great intensity, and who have little corresponding appreciation of pleasurable ones. Such a temper is partly due to natural disposition, and partly to individual circumstances. According to Caro (after von Hartmann), it is especially prevalent in periods of transition, in which old ways of thought have lost their hold, while the new order has not yet made itself fully known, or has not secured general acceptance for its principles. In such a state of things men's minds are driven in upon themselves; the outward order appears to lack stability and permanence, and life in general tends consequently to be estimated as hollow and unsatisfactory. Metchnikoff attributes the pessimistic temper to a somewhat similar period in the life history of the individual, viz.: — that of the transition from the enthusiasm of youth to the calmer and more settled outlook of maturity.