Freud described four crucial defining characteristics of sexual drives (1915a)

In the “Three Essays on the theory of sexuality”, Freud had already given what he had called a “provisional” definition of instincts: “By an “instinct” is provisionally to be understood the psychical representative of an endosomatic, continuously flowing source of stimulation, as contrasted with a “stimulus”, which is set up by single excitations coming from without. The concept of instinct is thus one of those lying on the frontier between the mental and the physical. […] What distinguishes the instincts from one another and endows them with specific qualities is their relation to their somatic sources and to their aims. The source of an instinct is a process of excitation occurring in an organ and the immediate aim of the instinct lies in the removal of this organic stimulus”.

Firstly, “By the pressure of an instinct we understand its motor factor, the amount of force or the measure of the demand for work which it represents. The characteristic of exercising pressure is common to all instincts; it is in fact their very essence. […] if we speak loosely of passive instincts, we can only mean instincts whose aim is passive”.

Secondly, “The aim of an instinct is in every instance satisfaction, which can only be obtained by removing the state of stimulation at the source of the instinct. In keeping with this notion of intermediate aims, in later passages of “Instincts and their vicissitudes”, Freud uses the term “aim” to refer, not to the satisfaction of instincts, but to the actions performed by the subject to reach satisfaction, e.g., to torture in the case of sadism, to watch in the case of scopophilia.

Thirdly, “The object of an instinct is the thing in regard to which or through which the instinct is able to achieve its aim. The object is not necessarily something extraneous: it may equally well be a part of the subject’s own body”.

It is what is most variable about an instinct and is not originally connected with it, but becomes assigned to it only in consequence of being peculiarly fitted to make satisfaction possible

Finally, “By the source of an instinct is meant the somatic process which occurs in an organ or part of the body and whose stimulus is represented in mental life by an instinct. We do not know whether this process is invariably of a chemical nature or whether it may also correspond to the release of other, e.g., mechanical, forces. The study of the sources of instincts lies outside the scope of psychology. Although instincts are wholly determined by their origin in a somatic source, in mental life we know them only by their aims. An exact knowledge of the sources of an instinct is not invariably necessary for purposes of psychological investigation; sometimes its source may be inferred from its aim”.

But although the ultimate aim of each instinct remains unchangeable, there e ultimate aim; so that an instinct may be found to have various nearer or intermediate aims, which are combined or interchanged with one another”

The importance of instincts in psychoanalytic theory, their still obscure character combined with the new opportunities to study SA offered by neuroimaging techniques starting in the nineteen nineties make of the instincts one of the objects “par excellence” of present and future neuropsychoanalytic investigation. Thus, the neuropsychoanalytic investigation of sexual instincts is motivated and made possible by the following conjunction: (i) in Freud’s own view, what makes the psychoanalytic theory of instincts particularly difficult to elaborate is the complex-both representational and somatic-nature of sexual instincts; and (ii) since a few years, the availability of functional neuroimaging techniques might shed light on the somatic, at least on the cerebral, aspects of sexual instincts.