In the works of my later years (_Beyond the Pleasure Principle_, _Group Psychology and the Analysis of the Ego_, and _The Ego and the Id_)[18] I have given free rein to the inclination to speculation which I kept down for so long and I have also taken stock of a new solution of the problem of the instincts. I have combined the instincts for self-preservation and for the preservation of the species under the concept of _Eros_ and have contrasted with it an instinct of death or destruction which works in silence. Instinct, in general, is regarded as a kind of elasticity of living things, an impulsion towards the restoration of a situation which once existed but was brought to an end by some external disturbance. This essentially conservative character of instincts is exemplified by the phenomena of the _compulsion to repeat_. The picture which life presents to us is the result of the working of Eros and the death-instinct together and against each other. It remains to be seen whether this construction will turn out to be serviceable. Although it arose from a desire to fix some of the most important theoretical ideas of psychoanalysis, it goes far beyond psychoanalysis. I have repeatedly heard it said contemptuously that it is impossible to take a science seriously whose most general concepts are as lacking in precision as those of libido and of instinct in psychoanalysis. But this reproach is based upon a complete misconception of the facts. Clear fundamental concepts and sharply drawn definitions are only possible in the mental sciences in so far as the latter seek to fit a department of facts into the frame of a logical system. In the natural sciences, of which psychology is one, such clearcut general concepts are superfluous and indeed impossible. Zoology and Botany did not start from correct and adequate definitions of an animal and a plant; to this very day Biology has been unable to give any certain meaning to the concept of life. Physics itself, indeed, would never have made any advance if it had had to wait until its concepts of matter, force, gravitation, and so on, had reached the desirable degree of clarity and precision. The fundamental concepts or most general ideas in any of the disciplines of science are always left indeterminate at first and are only explained to begin with by reference to the realm of phenomena from which they were derived; it is only by means of a progressive analysis of the material of observation that they can be made clear and can find a significant and consistent meaning.