'If a straight line falls on another straight line, the adjacent angles are together equal to two right angles'; 'If a body is unsupported, it falls'; 'If population increases, rents tend to rise': here 'if' means 'whenever' or 'all cases in which'; for to raise a doubt whether a straight line is ever conceived to fall upon another, whether bodies are ever unsupported, or population ever increases, is a superfluity of scepticism; and plainly the hypothetical form has nothing to do with the proof of such propositions, nor with inference from them.
Still, the disjunctive form is necessary in setting out the relation of contradictory terms, and in stating a Division (chap.

xxi.), whether formal (_as A is B or not-B_) or material (as _Cats are white, or black, or tortoiseshell, or tabby_).

And in some cases the hypothetical form is useful.

One of these occurs where it is important to draw attention to the condition, as something doubtful or especially requiring examination.

_If there is a resisting medium in space, the earth will fall into the sun; If the Corn Laws are to be re-enacted, we had better sell railways and buy land_: here the hypothetical form draws attention to the questions whether there is a resisting medium in space, whether the Corn Laws are likely to be re-enacted; but as to methods of inference and proof, the hypothetical form has nothing to do with them.
The propositions predicate causation: _A resisting medium in space is a condition of the earth's falling into the sun; A Corn Law is a condition of the rise of rents, and of the fall of railway profits_.
A second case in which the hypothetical is a specially appropriate form of statement occurs where a proposition relates to a particular matter and to future time, as _If there be a storm to-morrow, we shall miss our picnic_.