There are two possible explanations I would hazard (and likely there are more, which I ask that you'll inform me of, lumen, when you soon outstrip me in knowledge of Latin!). First a general point about grammar that I hope will lubricate your thinking about language. Skip to the bottom for easy answers.<br /><br />The subjunctive is probably the hardest part of Latin to learn -- and I should warn you that people generally say Latin becomes harder the more you know -- because it fits so awkwardly into grammatical rules. There is no way one can sum up the subjunctive and give it a root meaning, a "Grundbedeutung", like "doubt, imagination, possibility". I don't mean to discourage that as a rough-and-ready rule -- yes that will "explain" the subjunctive in about 80% of the cases; there will still remain 20% that falls weakly, if at all, beneath the rubric. We can make our method more sophisticated by adding more rules that account for our observations: perhaps we see a certain set of verbs always causing a subjunctive or other constructions supplying that mood. Perhaps we can account for 99%; we will never account for 100% because the problem is like trying to solve for pi by calculating the areas of polygons with successively more sides (Antiphon's method, I think?). That was probably a poor metaphor, I admit. In essence language is a phenomenon that, from the outside, we can at best approximate by rules.<br /><br />Yes but what does this all mean for me? The coverage of the best Latin grammar book about the subjunctive will always be inadequate compared to a native speaker's command of the language, and perhaps even compared to a sensitive reader's who has spent years with Latin. There will be instances where the subjunctive is obviously right, then some where it's obviously wrong, and gray areas where exposure to lots of usage is the only way to catch the appropriateness of the subjunctive, and its particular meaning in that context. <br /><br />This ambiguity and resistance to strict pigeon-holing applies very well to conditional sentences. We know fully the regular divisions into more vivid or less vivid, etc. But there are fairly common sentences by respectable Latin stylists which break these rules:<br /><br />Si reviviscant et tecum loquantur, quid talibus viris responderes? Cicero, Fin., IV. 22, 61<br /><br />I don't think this sentence is all that hard to hear, even though the tenses don't make strict sense. One could think of it as switching the tone of the sentence halfway through, a possibility becoming more contrary to fact. This is truly illogical, but perfectly expressive. In this case the rules about conditionals are exposed for what they are: guidelines.<br /><br />
<br /><br />So, after that extensive and I'm not sure strictly necessary preamble, my first possible answer is that Quintilian is beginning a simple or Logical condition (terminology differs; I mean a conditional that takes the indicative across protasis to apodosis) and subsequently shifts conditions to dramatically emphasize the generality of the condition. This has basically been the direction of some good explanations so far attempted. <br /><br />Another, and I think slightly closer, take on the sentence is that the sentence is entirely a Logical condition. That is, the sentence expresses a factual condition in no uncertain terms: if this, then definitely that. Only, what must be converted into the apodosis is a jussive/hortatory subjunctive (a command, almost a wish "let him know...!"). In these instances one does not level off the subjunctive into an indicative, but keeps it subjunctive for the sake of vividness.<br /><br />Since the subjunctive is so hard to "get", it often comes down to hearing it as Latin and then deciding based on context what tone the sentence is getting at. This takes exposure. When someone says that subjunctive often expresses doubt, imagination, or possibility he is suggesting good ways that often describe the tone of the subjunctive (which is not the same as giving sufficient, grammatical cause!). One fact which may help us catch the tone of Quintilian's sentence. Quintilian was rabidly pro-Cicero, so there's very little chance that this is spoken ironically, sarcastically, or with doubt in mind. Hope is close but insufficient (as it's been observed the hope is slightly misplaced, since the hope is probably that the student enjoy cicero, not that he know he has advanced); something like vividness or a tinge of more intense emotion perhaps. The sense of future is also slightly involved. In my opinion it would be rendered best into English if one used the future and added an exclamation point:<br /><br />If anyone likes Cicero's books, he'll know that he's advanced!<br /><br />My suggestions are convoluted, I know, and are phrased in terms of the grammatical rules that I just criticized. But it's all worth it if by using these cudgel-like grammatical categories we can get closer to the meaning of Quintilian.