A hypothesis is a probable supposition. But any wild guess should not be treated as hypothesis.
A hypothesis should possess a higher degree of probability. Newton makes a remark on certain hypotheses by saying “hypothesis non-fingo”. It stands for the expression “I do not form hypothesis”.
It signifies that unless the degree of probability is high, there is no meaning to take any stray supposition to be a hypothesis.
It shows that if a hypothesis lacks the higher degree of probability, it is not considered as a legitimate hypothesis. For this reason logicians have suggested some broad conditions for considering a hypothesis to be legitimate.
i) A legitimate hypothesis cannot be framed with a self-contradictory statement or absurd expression. It should be definite and specific.
All self-contradictory statements are meaningless statements. It is never possible that with the help of self-contradictory statement any fact can be justified.
It is to say that the statement of hypothesis should be sensible, neither meaningless nor absurd. The hypothesis should be precise, definite and not vague.
ii) The hypothesis should be adequate. A hypothesis is required when there is demand of explanation in respect of some problematic event. If the hypothesis is not adequate it cannot explain the event correctly.
In order to become adequate the hypothesis should be framed after taking all the connected known truths into account. For example, in case of a theft without considering any other fact if someone says that the servant is the thief then it is not adequate.
In a complex situation the possible reasons thought of may be quite insufficient or quite irrelevant to explain the fact. If we say that a political leader is responsible for our social evils here it is not clear in what way he is responsible for the effect. If we say that the city plan is responsible for the social evils here it becomes an irrelevant conjecture.
So the hypothesis should be adequate in the sense that there must be adequate basis of framing the hypothesis and the basis is the capability of explaining the facts (effects).
iii) The hypothesis should be verifiable. Since it is only a provisional supposition the verifiable aspect of the hypothesis is very important. No hypothesis can be accepted either as the cause or as a law unless it is verified.
The verification may be done in various ways. The hypothesis is compared with other established truths or facts. Or certain conclusions are deduced from the hypothesis and the coherence of the new conclusion with other established facts is examined.
Verification helps in reaching higher degrees of probability by gradual confirmation of the hypothesis. Without verification a hypothesis has no place in scientific procedure and is useless.
iv) It should usually be consistent with previously established truths or laws:
The hypothesis should not be framed going against the established laws of nature, unless there is the need of an adhoc hypothesis. There are certain laws of facts which are normally unchallengeable in nature. For example, every object is extended is an accepted truth.
Similarly, like begets like, living beings are mortal, a heavy object does not hang in the air because of the force of gravitation etc. are the exceptionalness regularities of nature.
They are so to say the laws of the natural world. Any supposition, that violates any such accepted truths becomes itself unacceptable.
It does not mean that no law of science or the law of society cannot be challenged. But it is suggested that unless there is sufficient ground, a hypothesis should not be framed going against the established truths.
v) A hypothesis should be based on Vera causa or real cause. This point suggests that one should be careful against superstitions while framing hypothesis.
In this regard the remark of Newton is quite significant that “only real causes are to be admitted in explanation of phenomena”.
This remark was made to guard against blind beliefs while investigating the cause. For example, sometimes it was thought that the drought occurs due to some supernatural power; or diarrhoea is due to the anger of the village goddess; etc.
These suppositions are no doubt unrealistic. So, it is suggested that hypothesis should be realistic or based on Vera causa.
vi) A’ hypothesis should have the power of prediction. A hypothesis should explain not only the present event, but also be capable to explain future events of the same type as, well.
Then only the veracity of the hypothesis can be accepted. After knowing the cause of solar or lunar eclipse the scientists can make accurate prediction when the next eclipse will occur. On the basis of the predictive power, a hypothesis is accepted as a legitimate one.
vii) A hypothesis should be simple. A simple hypothesis does not involve too much of assumptions. When there are two incompatible hypotheses, the one that is comparatively simpler is accepted.
In respect of the path of the planets two hypotheses, namely, the Ptolemic and the Copernican, were offered. Further observations confirmed the hypothesis of Copernicus to be simpler than that of Ptolemy. So it was accepted. Hence, a hypothesis should be simple.