Kommentar zu Dewarim 7:17

כי תאמר בלבבך — You must admit that the word כי here denotes “perhaps": perhaps thou wilt say in thy heart, “Because they are many, I shall be unable to dispossess them". Do not speak thus, לא תירא מהם THOU SHALT NOT BE AFRAID OF THEM. — For it is not possible to explain it in one of the three other meanings of כי so that the words לא תירא מהם will then appropriately fit in with it.

כי תאמר...איכה אוכל להורישם, לא תירא מהם, for you will say that they outnumber you. Do not say this in a manner which will inspire fear in you, but reflect on the fact that this would be an argument only if the Lord your G’d had not already demonstrated that with His help you can overcome. זכור תזכור את אשר עשה ה' אלוקיך לפרעה וכל מצרים, they outnumbered you by a far greater margin than the Canaanites and they were stronger than the Canaanites.

And it is impossible to explain [the word כי] according to any other meanings of, etc. In other words, the word כי cannot mean, “because,” or, “rather,” for these terms cannot begin an idea. The word כי also cannot mean, “if,” for it is not possible to say, “If you will say [i.e., think] this, then do not think it.” For if you have already thought it, it is not possible to say that you should not think it. Furthermore, [if the word כי means “if”] then the verse would imply: If you think… then do not fear them. But if you do not think… then fear them. But this is not correct. Rashi explains in this manner in Gittin (90a).