Amongst my patients suffering from shell-shock, I have had very many hundreds who have completely forgotten some of the most unpleasant and terrifying experiences which occurred to them out at the front. Others unconsciously had found the easiest method of dealing with the unpleasant past to be that of blotting the whole of it out, dissociating it completely from their conscious mind, and then stating that they remembered nothing of their lives until they woke up in hospital. It is not only memories, however, which are repressed and remain dormant in the unconscious mind. Most of our primitive instincts handed on from our savage forefathers before even the evolution of man in his present form, lie similarly buried in this unconscious part of the mind, and we are wont to deny emphatically that we possess these unpleasant instincts. Nevertheless, just as _in utero_ we repeat more or less in detail the history of our physical evolution, so do we at that period and in childhood repeat to a great extent the history of our psychic evolution; and just as during this early period we possess the physical attributes of many of our ancestors, such as the gills of the fish or the tail of the lower vertebrates, so psychically do we at a somewhat later period, possess the instincts and desires of our progenitors, and utilise them as the hidden foundation stones in building our adult mental constitution. These various primitive instincts include all kinds of desires which would consciously be regarded as sexual perversions and moral crimes of different kinds, and they are present in all of us without exception. Our upbringing and conscious outlook upon them, however, causes them to be so abhorrent to us, that we successfully keep the majority of such ideas and feelings _from ever coming out of the unconscious in their primitive form_. In other words, we repress them. Occasionally, however, there is a tendency for these ancestral instincts to become conscious, and in our further efforts to prevent this we may develop instead hysterias, obsessions and unreasonable fears, together with many other nervous and abnormal signs and symptoms, into the nature of which it is not my intention to inquire further in this present volume. Those who are interested in pursuing this line of investigation will find an elementary account of it in a previous work of mine, “The Elements of Practical Psycho-Analysis.” All that I wish to emphasise here is that we do push out from the conscious mind unpleasant thoughts and memories, that we do repress and keep in the unconscious mind unpleasant desires and instincts, and that we do, as a result of this, have many unconscious or semi-conscious conflicts within ourselves, which may lead to unpleasant feelings of depression, irritability, fear, or in more pronounced cases hysterias, obsessions, and even permanent mental derangement. §2