The personal and scientific environments within which Freud grew up therefore represent his primary influences. A further influence came in the form of physics. The second half of the nineteenth century, during which Freud did most of his important work, saw great advances in physics. According to Thornton, the discovery mostly responsible for this was Helmholz's principle of conservation energy. Helmholz held that the total amount of energy in a physical system is constant; that it could be changed but not annihilated; and that when the energy is moved from a part of the system, it would reappear in another part. This principle influenced areas such as thermodynamics, electromagneticism, and nuclear physics. The 19th century therefore saw major discoveries that changed the world.

For Freud, this meant that his field of study was significantly influenced by the principle. At the University of Vienna, for example, Freud's professor, Ernst Brucke published...
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