Juxtaposition is an act or instance of placing two elements close together or side by side. This is often done in order to compare/contrast the two, to show similarities or differences, etc.
Juxtaposition in literary terms is the showing contrast by concepts placed side by side. An example of juxtaposition are the quotes "Ask not what your country can do for you; ask what you can do for your country", and "Let us never negotiate out of fear, but let us never fear to negotiate", both by John F. Kennedy, who particularly liked juxtaposition as a rhetorical device.Jean Piaget specifically contrasts juxtaposition in various fields from syncretism, arguing that "juxtaposition and syncretism are in antithesis, syncretism being the predominance of the whole over the details, juxtaposition that of the details over the whole". Piaget writes:
In visual perception, juxtaposition is the absence of relations between details; syncretism is a vision of the whole which creates a vague but all-inclusive schema, supplanting the details. In verbal intelligence juxtaposition is the absence of relations between the various terms of a sentence; syncretism is the all-round understanding which makes the sentence into a whole. In logic juxtaposition leads to an absence of implication and reciprocal justification between the successive judgments; syncretism creates a tendency to bind everything together and to justify by means of the most ingenious or the most facetious devices.
In grammar, juxtaposition refers to the absence of linking elements in a group of words that are listed together. Thus, where English uses the conjunction and (e.g. mother and father), many languages use simple juxtaposition ("mother father"). In logic, juxtaposition is a logical fallacy on the part of the observer, where two items placed next to each other imply a correlation, when none is actually claimed. For example, an illustration of a politician and Adolf Hitler on the same page would imply that the politician had a common ideology with Hitler. Similarly, saying "Hitler was in favor of gun control, and so are you" would have the same effect. This particular rhetorical device is common enough to have its own name, Reductio ad Hitlerum.