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What is the difference between 'speculative', 'hypothetical' and 'conjectural'?

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+1 for the interesting question. You could have thrown in conjectural for good measure. – Alain Pannetier Φ May 9 '11 at 6:05
up vote 4 down vote accepted

A hypothesis in a scientific sense is an idea based on existing evidence, that can be tested. Speculation isn't necessarily based on anything, and may not be provable.

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+1. In everyday talk, you will often find hypothesis qualified by reasonable (a reasonable hypothesis) whereas the adjective often attached to speculation is actually wild ("a wild speculation"). – Alain Pannetier Φ May 9 '11 at 6:21
@ Alain: I could understand the phrase 'a reasonable hypothesis', but what is going to be the exact meaning of 'wild speculation'? I just looked up a dictionary, but couldn't determine the meaning of 'wild', which has several meanings. – czh May 9 '11 at 7:02
@ Alain: I think I got the meaning of 'wild' in the context of 'wild speculation'. 'wild' means 'rough', right? – czh May 9 '11 at 7:17
Sam's answer is the most accurate, I pretend to say that has a 100% accurate approach – Lefteris Gkinis May 9 '11 at 7:50
@czh, in this context 'wild' means remotely plausible. You can understand this sense by comparing it to reasonable: a wild person/behaviour is the opposite of a reasonable one. The same applies to hypothesis a wild speculation is unfounded. For instance wildly speculative gossips about celebrities: with little or no supporting evidence. – Alain Pannetier Φ May 9 '11 at 8:18

Speculative is based on arbitrary assumptions, while hypothetical is based on certain priori reasoning or could also be imaginative.

Update: 1

Speculative is often used in context of betting and waging:

His investments in crude oil are speculative - they are based on assumptions that middle east crisis may further intensify.

Generating something hypothetical is a good tool in scientific research, proposals and education:

Professor, while teaching to his students, said, "Imagine a hypothetical situation where you find five bills of $100 lying around..."

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Does it mean that 'speculative' is more general than 'hypothetical'? If so, would you give me an example where speculative is OK, but hypothetical may not be proper? – czh May 9 '11 at 5:31
@chz: Updated the answer, I guess that should help. – check123 May 9 '11 at 5:57
@chz: It is hard to find a situation where 'hypothetical' and 'speculative' may be used interchangeably at first place; they are often in used different contexts. – check123 May 9 '11 at 5:59

No one seems to have addressed "conjecture", which seems to be midway between "hypothetical" and "speculative". It is an opinion or idea based on limited/incomplete information (Merriam-Webster.com and thefreedictionary.com). So, one might begin with some wild speculation, which after some brainstorming leads to a conjecture, having one gather all available information to form a hypothesis.

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Commonly, a "hypothetical" (as a noun) is something we know to be false and are supposing to examine its consequences. A claim is "hypothetical" (as an adjective) if it can serve as a hypothesis, that is, if is a claim that can be tested.

Something "speculative" is something that we may have some reason to suspect is true but it's not sufficient to justify a belief that it's true. It also has a specialized use in finance to suggest an investment that has moderate or greater than usual risk.

Something "conjectural" is something that someone proposes as truth or suggests may be true without sufficient evidence to justify believing that it is true.

"I know you have no children, but consider this hypothetical: What would you do if your daughter ..."

"We have little evidence that it will work, it's purely speculative."

"You say he is trustworthy, but isn't that just conjectural given we have no good reason to trust him?"

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