921: Hypothesis versus Hypothetical (Lexical Classes) Jun 17, 2017

In general, a word's derivatives make it very easy for a speaker to apply one sort of concept throughout different parts of speech, also called 'lexical classes'. Some words, though theoretically all words, can be used in different contexts in English to change the part of speech to which it belongs without affixes or anything else e.g. 'water' in 'I drink water' and 'I water the plants is acting as a noun and verb respectively. Much of the time, either for clarity or tradition, speakers will modify a word to convey the same sort of concept but within lexical classes, such as with 'analysis', a noun, 'analyze', a verb, and 'analytical', an adjective. In a few instances, the derivatives have changed meanings however, so while 'hypothesis' and 'hypothesize' both relate to the explanation of something based on minimal evidence usually in regard to research, 'hypothetical' and 'hypothetically' are used in a less formal way to mean anything possibly true but not necessarily, and would not relate to some kind of proposed analysis. To be especially clear in this case, one could opt to say 'by hypothesis', because in this case the derivative has gained an additional and slightly conflicting sense.