Simplicity is the state or quality of being simple. Something easy to understand or explain seems simple, in contrast to something complicated. Alternatively, as Herbert A. Simon suggests, something is simple or complex depending on the way we choose to describe it. In some uses, the label "simplicity" can imply beauty, purity, or clarity. "Simplicity" may also occur with negative connotations to suggest a deficit or insufficiency of nuance or of complexity of a thing, relative to what one supposes as required.
According to Occam's razor, all other things being equal, the simplest theory is most likely true. In other words, simplicity is a meta-scientific criterion by which scientists evaluate competing theories.
A distinction is often made[by whom?] between two senses of simplicity: syntactic simplicity (the number and complexity of hypotheses), and ontological simplicity (the number and complexity of things postulated). These two aspects of simplicity are often referred to as elegance and parsimony respectively.
John von Neumann defines simplicity as important esthetic criteria of scientific models:
[...] (scientific model) must satisfy certain esthetic criteria - that is, in relation to how much it describes, it must be rather simple. I think it is worth while insisting on these vague terms - for instance, on the use of word rather. One cannot tell exactly how "simple" simple is. [...] Simplicity is largely a matter of historical background, of previous conditioning, of antecedents, of customary procedures, and it is very much a function of what is explained by it.
Simplicity is a theme in the Christian religion. According to St. Thomas Aquinas, God is infinitely simple. The Roman Catholic and Anglican religious orders of Franciscans also strive for personal simplicity. Members of the Religious Society of Friends (Quakers) practice the Testimony of Simplicity, which involves simplifying one's life to focus on what is important and disregard or avoid what is least important. Simplicity is tenet of Anabaptistism, and some Anabaptist groups like the Bruderhof, make an effort to live simply. 
A distinction is often made between two fundamentally distinct senses of simplicity: syntactic simplicity (roughly, the number and complexity of hypotheses), and ontological simplicity (roughly, the number and complexity of things postulated). [...] These two facets of simplicity are often referred to as elegance and parsimony respectively. [...] The terms 'parsimony' and 'simplicity' are used virtually interchangeably in much of the philosophical literature.
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