Freud's Psychoanalytical Theory
When Charles Darwin's book, "On The Origin of Species", was published, the general and scientific public began formulating a whole new perception of man, how it is part of the animal kingdom and not a superior nor inferior race. This made it possible to include Homo Sapiens as part of scientific investigation, including the behavioral aspect. Sigmund Freud was four years old at the time, but as he grew older, his curiosity and intuition expanded to this behavioral investigation of man, and he accepted and embraced it. This is only a side note, a precursor to his psychoanalytical theory; but it was his inspiration.
Around the late 1800s was when Freud was truly inspired to create his psychoanalytical theory, where Helmholtz formulated his principle on energy conservation. This principle states that, when in effect, energy is constant, and it can be manipulated, but not annihilated. It also states that if energy is moved from one part of a system, it must reappear in another. Ernst Bruck, whom Freud worked under, published a book in 1874 stating that all living organisms are essentially energy systems to which the principle of energy conservation applies. Freud, who admired Bruck and his works, quickly analyzed this principle, and "broke the ground" on psychoanalytical studies by stating that the principle on energy conservation can be applied to "psychic energy". "Psychic energy" states that the human personality and mentality is an energy system; that is is the purpose of psychology to investigate the function of the human personality and take note on its modifications, transmissions, and conversions. This perception is the cornerstone on his psychoanalytical theory.