Probable, a.
Etymology: [L. probabilis, fr. probare to try, approve, prove: cf. F. probable. See Prove, and cf. Provable.]
1. Capable of being proved. [Obs.]
2. Having more evidence for than against; supported by evidence which inclines the mind to believe, but leaves some room for doubt; likely. That is accounted probable which has better arguments producible for it than can be brought against it. South. I do not say that the principles of religion are merely probable; I have before asserted them to be morally certain. Bp. Wilkins.
3. Rendering probable; supporting, or giving ground for, belief, but not demonstrating; as, probable evidence; probable presumption. Blackstone. Probable cause (Law), a reasonable ground of presumption that a charge is, or my be, well founded. -- Probable error (of an observation, or of the mean of a number), that within which, taken positively and negatively, there is an even chance that the real error shall lie. Thus, if 3" is the probable error in a given case, the chances that the real error is greater than 3" are equal to the chances that it is less. The probable error is computed from the observations made, and is used to express their degree of accuracy. -- The probable, that which is within the bounds of probability; that which is not unnatural or preternatural; -- opposed to the marvelous.
Shades of meaning
Fernald has no entry of its own for probable; he lists it under apparent, likely.
Where he writes about it:
Probable implies that we know facts enough to make us moderately confident of it. — from apparent
Seeming expresses great doubt of the reality; seeming innocence comes very near in meaning to probable guilt. — from apparent
Apparent indicates less assurance than probable, and more than seeming. — from apparent
A man's probable intent we believe will prove to be his real intent; his seeming intent we believe to be a sham; his apparent intent may be the true one, tho we have not yet evidence on which to pronounce with certainty or even with confidence. — from apparent