|Diagram of induced movement (photon-mediated) dynamics (conformational change) of the retinal molecule C20H280 located in the retina of the eye.|
The literal meaning of motivation or drive, according to American gifted-children psychologist Florence Goodenough, is "the process of induced movement."  The description of "induced movement", in small molecules, such as the famous "walking molecules" of American physical chemist Ludwig Bartels, is relatively simple to explain. The description of induced movement for "human molecules", however, is a more difficult description. The basic model, in both cases, can be understood by the description of photon-induced movement in 3-element retinal molecule. 
The basic model for induced movement or "molecular life", is the movement dynamics involved in the life of the 3-element retinal molecule C20H28O, a light sensitive molecule found in the retina of the eye. If the energy contained in a single photon is of the correct wavelength, between 400 and 700 nm, it will function to break what are called pi-bonds found between the eleventh and twelfth carbon atoms near the kink in the structure of the retinal molecule. When these pi-bonds break, this ‘forces’ the retinal molecule to rearrange into a straightened configuration:
This basic model, in which a molecule is forced to react, i.e. moves dynamically, to a photon or field particle stimulus, is the basic model (poster child) for human molecular life, i.e. for all human behavior. The human molecule, a 26-element molecule, is no different than the retinal molecule, a 3-element molecule; a fact which can be deduced through study of a molecular evolution table. Human chemical reactions will always be exact and repetitive, similar to the bending and straightening actions of the simple retinal molecule. More to the point, the central nervous system of the retinal molecule is no different, complexity aside, than the central nervous system of the human molecule: each CNS is comprised, at its core, of valence shell electron-photon interactions.
This photon induced retinal-bending mechanism, to note, was discovered in 1958 by the American biochemist George Wald and his co-workers; work for which Wald won a share of the 1967 Nobel Prize in Physiology or Medicine with Haldan Keffer Hartline and Ragnar Granit. 
1. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview). Morrisville, NC: LuLu.
2. Young, Paul T. (1936). Motivation of Behavior – the Fundamental Determinants of Human and Animal Activity, (ch. 2: “The Energetics of Activity”, pg. 70-73. New York: Wiley.
3. Thims, Libb. (2008). The Human Molecule, (section: "Free will", pgs. 59-61). (preview). Morrisville, NC: LuLu.
4. Nobel Prize in Medicine 1967
5. Mahon, basil. (2003). The Man Who Changed Everything - the Life of James Clerk Maxwell. Hoboken, NJ: Wiley.