Thesis and antithesis can be distinguished because they tend to involve completely opposed or conflicting ideas. They are used in many different settings, such as in literature and philosophical discourse. In some cases, they are used to make a point about an idea in a single comment or phrase, while in others, the two are sustained throughout an entire work, as when two characters are shown to have completely opposed characteristics over the course of a story. In philosophy, they are often combined into a synthesis that provides a new way of looking at the world.
Opposition is the defining feature that distinguishes thesis from antithesis in any given work. The two are commonly used in literature to demonstrate the opposition between two different ideas, actions, or characters. When Neil Armstrong first set foot on the moon, for instance, he described the action as "one small step for (a) man; one giant leap for mankind." This use demonstrates the opposition inherent in the fact that, while Armstrong's physical action was only a single step by a single man, it represented a tremendous leap forward and a momentous occasion for celebration on the part of mankind as a whole.
Writers may use thesis and antithesis in many different ways and for a variety of different reasons. In some cases, the techniques are used to demonstrate that two ideas, actions, or objects that appear unrelated are, in fact, inherently opposed to each other. In other cases, it may be used to point out contradictions in ideas that appear, upon casual examination, to agree with each other. In literature, the two are often embodied in characters. Two characters, such as a main character and a main villain or a character and his foil, are shown to embody ideas and to possess traits that are opposed at a fundamental level.
Thesis and antithesis also are often used in philosophical discussions in order to reach new conclusions about accepted ways of thinking. First, the thesis or accepted way of thinking or acting, is expressed. Next, an antithesis is proposed that demonstrates conflicts or problems with the original thesis. The third step is referred to as synthesis. The thesis and the problems and objections are combined in order to form a new worldview that more effectively or logically suggests a new manner of thought or action.