It's amusing to observe mankind sometimes try to heed, wittingly or not, Thoreau's injunction to "Simplify!" Since specialized inquiry favors complexity over simplicity--as in, "Yes, but"" "On the other hand""---there is always the temptation to cut through the clutter in order to arrive at an all-encompassing explanation for everything. Parmenides long ago theorized that all is one, and, more recently, physicists are on the trail of a grand unified field theory. The beauty of a monocausal explanation of what makes us tick is that all evils can be attributed to some basic flaw behind the four usual suspects of power, money, lust, revenge.
Christianity started the discussion with its useful formulation of Original Sin. Derived from the Judaic concept of "the evil impulse," broached by Paul and refined by Augustine, it neatly explains the ubiquity of mendacity and cruelty in people, as well as the resulting horror show that is human history. The Fall of the first human couple also allowed thinkers in the Middle Ages to explain why everything beneath the moon (in the Ptolemaic, human-centric universe) is mutable and mortal even though everything on the moon and above is perfect and unchanging.
The first glimmers of secularism during the Renaissance begot a less Biblically-based explanation. Thomas More, in his Utopia, came upon a more modest version of a comprehensive formula: Pride. He calls it the one "single monster, the prime plague and begetter of all others." In other words, the error which the human species commits in thinking that the world revolves around the earth is duplicated in each individual's thinking that it revolves around himself. This hypothesis was echoed two centuries later by Jonathan Swift, who was outraged over seeing the human being, this "Lump of Deformity and Diseases both in Body and Mind, smitten with Pride"; without taking Pride into account, he asserts, one cannot "thoroughly understand Human Nature."
A different name was accorded to this human flaw by More's contemporary, Erasmus: Folly. This female "divinity" asserts that she "maintains dominion over all things, and rules even Emperors," that she is first among gods because she "confers all benefits on all men." Obviously the two concepts are related, pride being a form of folly. Under either name, human nature is not malicious or depraved--as Christianity would have it--but rather ignorant, stupid, thoughtless, inconsistent. It notably makes choices according to the pleasure principle, which satisfy immediate needs at the expense of medium and long term unforeseen consequences. Like Oedipus or Jesus we thus bring about, often with the best intentions, the very disasters we seek to avoid. We neither know what is good for us, nor, even if we did, how to find it.
If Pride and Folly are sometimes hard to distinguish, that is not the case with some new monocausal explanations offered by nineteenth century thinkers. Darwin's theory of evolution, as it applies to humanity, is a matter of the unconscious all-encompassing drive for self preservation, both of individual and of species; it--and not original sin or pride or folly--shapes our many actions, down to the choice of mate we fall in love with in pursuit of procreation.
Marx shifts the discussion from biology to economics. His philosophical formulation is that "the mode of production of material life determines the social, political and intellectual life process." In simple terms, he who has the money calls the tune, and life is thus explained as a secular version of radix malorum est cupiditas. Behind all the rhetoric, the pieties, and the proclaimed ideals stands, basically, universal greed.
Money, of course, is closely related to power, which it both needs and produces, but for Nietzsche the true sequence is reversed. Power is central, and money but an accessory. "The really fundamental instinct of life aims at the expansion of power".Our entire instinctive life [is] the development and ramification of one basic form of the will--namely, of the will to power." Even a meek and ascetic clergyman may seem otherworldly, but his goal is power all the same, for power over men's mind trumps power over their mere bodies or wallets.
Yet another dimension is introduced by Freud. If Darwin emphasizes self preservation, the sexual component of that drive becomes an end in itself in Freud's thinking. "The manner in which the sexual impulses can thus be influenced and diverted enables them to be employed for cultural activities of every kind".Powerful components are acquired for every kind of cultural achievement by this diversion of sexual instinctual forces from sexual aims." All activities are but a channeling--a sublimation--of the frustrated eros into activities whose sexual roots are hidden. If Marx sees society as distorted by greed, Freud sees civilization as made possible by unsatisfied lust. Thus a man obsessed with making money does not prove Marx to be right, because he has been deflected from the central goal, sex, or because accumulated money brings him to that goal--easy access to women. Indeed, one might note that the behavior of many politically ambitious men who recklessly disabled their drive for higher office because of an inability to resist sexual temptation suggests that Freud may be more accurate than Marx or Nietzsche.
Nor should we ignore yet two other simplifying theories. Turgenev's belated Enlightenment hero, Bazarov, regards reason and science as the only reality: "What's important is that two times two makes four; all the rest's nonsense." His fictional contemporary, Dostoyevski's Romantic/Modernist Underground Man, by contrast, singles out "spite" as the universal power in human affairs (an offshoot of Original Sin and father of Nietzsche's ressentiment?). How else explain that Adam, given sway over nearly everything, proceeds to do the one thing he was mandated not to do? Take that, God!
The irony of this brief survey is that it establishes, by the sheer multiplicity of monocausal explanations, the fact that no single one is sufficiently persuasive. Voltaire said that, where there are no sects, such as on the Pythagorean theorem, we have the truth; where we have sects, we have no truth. Thus does the quest for a monocausal explanation, by becoming ensnared in the diversity of human insights and assertions, destroy itself. What was supposed to be singular and dispositive turns out to be fragmentary, and the inquirer ends where he began, with a multiplicity of explanations, each of which seems to possess a kernel of truth.
Sorry about that, Mr. Thoreau.