A supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation.
Philosophy: A proposition made as a basis for reasoning, without any assumption of its truth.
The key part of this definition is the phrase “limited evidence,” though this requirement is more subjective when used in philosophy (see Philosophy). There needs to be a reasonable basis for anticipating an idea to be true, and some concept of how the idea can be tested.
A baseless concept is not a hypothesis. For example, the idea that Canada exists in another dimension is not a hypothesis since it is wholly speculative. On the other hand, a consideration that the moon is made of swiss cheese, based on the fact that it appears light in color and looks to have holes (initial evidence from observation), is a hypothesis.
Once a hypothesis has been stated we then apply experimentation. If early repeated experiments are negative then the hypothesis is done, assuming that the experiments were properly done. However, this failure may be taken into account and a new hypothesis formed. In the moon hypothesis, ((if our experiment showed that the surface temperature is high, thus cheese would not be a solid at such temperatures,)) we can alter our idea and consider an alternate element able to withstand high temperatures. We could next speculate that moon is made of soft powder. Repeat and refine.
If experiments are positive, that is, results are consistent with the hypothesis being true, then further experiments are indicated, both to replicate the result by other experimenters (to fa