_If_, too, may precede propositions wherein there is no doubt whatever implied: in other words it may be used instead of _since_. In some languages this interchange goes farther than in others; in the Greek, for instance, such is the case with εἰ, to a very great extent indeed. Hence we must look to the meaning of the sentence in general, rather than to the particular conjunction used. It is a philological fact that _if_ may stand instead of _since_. It is also a philological fact that when it does so it should be followed by the indicative mood. This is written in the way of illustration. What applies to _if_ applies to other conjunctions as well. § 511. As a point of practice, the following method of determining the amount of doubt expressed in a conditional proposition is useful:-- Insert, immediately after the conjunction, one of the two following phrases,--(1.) _as is the case_; (2.) _as may or may not be the case_. By ascertaining which of these two supplements expresses the meaning of the speaker, we ascertain the mood of the verb which follows. When the first formula is the one required, there is no element of doubt, and the verb should be in the indicative mood. _If_ (_as is the case_), _he _is_ gone, I must follow him_. When the second formula is the one required, there _is_ an element of doubt, and the verb should be in the subjunctive mood. _If_ (_as may or may not be the case_) _he _be_ gone, I must follow him_. § 512. The use of the word _that_ in expressions like _I eat that I may live_, &c., is a modification of the subjunctive construction, that is conveniently called _potential_. It denotes that one act is done for the sake of supplying the _power_ or opportunity for the performance of another. The most important point connected with the powers of _that_ is the so-called _succession of tenses_. § 513. _The succession of tenses._--Whenever the conjunction _that_ expresses intention, and consequently connects two verbs, the second of which takes place _after_ the first, the verbs in question must be in the same tense. I _do_ this _that_ I _may_ gain by it I _did_ this _that_ I _might_ gain by it. In the Greek language this is expressed by a difference of mood; the subjunctive being the construction equivalent to _may_, the optative to _might_. The Latin idiom coincides with the English. A little consideration will show that this rule is absolute. For a man _to be doing_ one action (in present time) in order that some other action may _follow_ it (in past time) is to reverse the order of cause and effect. To do anything in A.D. 1851, that something may result from it in 1850 is a contradiction; and so it is to say _I _do_ this_ that _I _might_ gain by it_.