What does Machiavelli mean on page 29 of The Prince?

Question

What does Machiavelli mean when he says the following in The Prince?

he must be a very true friend, or a thoroughly determined enemy of the prince, to keep faith with you.

It's on page 29 of this PDF version of the text (in chapter 19).

Particularly I am still unsettled about “to keep faith with you” at the end. What is this adverbial phrase referring to? Because if it is regarding “he must be a very rare friend” then it makes sense. But if it is regarding “a thoroughly obstinate enemy of the prince” (which is more likely here because that is the phrase comes right before “to keep faith you”) then it just sounds awkward…

Answer 1

The context is a paragraph discussing conspiracy against a prince (i.e. ruler). When a conspirator looks for support from other people, anyone who gets involved in the conspiracy is constantly at risk of being betrayed to the ruler in return for a reward. This is what Machiavelli in the words preceding the quote in the question:

As soon as you have opened your mind to a dissatisfied person, you have given him the material with which to satisfy himself, because by informing on you he can look for every advantage. So, because of the certain gains from informing and the uncertain gains and certain dangers from conspiring, (...)

So in order to be loyal to you or to support you (see the synonyms for keep faith with in Collins), a co-conspirator

must be a very true friend [i.e. of you], or a thoroughly determined enemy of the prince to keep faith with you [i.e., you as a conspirator, after you have revealed your intentions].

For comparison, here is the relevant passage in Peter Bondanella's translation (Oxford University Press, 2005, 2008):

Anyone who conspires cannot act alone, (...). As soon as you have revealed your intention to one malcontent, you give him the means to make himself content, since he can have everything he desires by revealing the plot. This is so much the case that, seeing a sure gain on the one hand, and one that is doubtful and full of danger on the other, if he is to remain loyal to you he must either be a rare kind of friend or a wholly obstinate enemy of the prince.

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Response to a comment saying that Machiavelli is here giving advice to the prince:

It is important to be aware of the fact that the pronoun "you" in The Prince does not always refer to the ruler (the "prince"). This becomes obvious when you substitute "the prince" for "you" in this whole passage (and adapt "your" etc accordingly):

A conspirator cannot act alone, nor can he take a companion except from those whom he believes to be dissatisfied. As soon as the prince [has] opened his mind to a dissatisfied person, he [has] given him the material with which to satisfy himself, because by informing on the prince he can look for every advantage. So, because of the certain gains from informing and the uncertain gains and certain dangers from conspiring, he must be a very true friend, or a thoroughly determined enemy of the prince, to keep faith with the prince.

This obviously does not make sense, since "informing" here refers to revealing the conspiracy to the prince. "You" here refers to someone who is thinking of starting a conspiracy against the prince.

Answer 2

You seem to be assuming that “to keep faith with you” must refer either to “he must be a very rare friend” or to “a thoroughly obstinate enemy of the prince”. Actually, it refers to both. Let me simplify the sentence structure for a moment:

• He must be X, to do Y.

  This sentence means that someone who does Y is necessarily X. If someone does Y, then they must be X. Now let's replace the single particle X by an either-or possibility:

• He must be A or B, to do Y.

  This has exactly the same meaning as above, except that X is replaced by "A or B". It means that if someone does Y, then they must be either A or B - either they must be A, or they must be B.

In this case, Y is "keeping faith with you", A is "a very rare friend [to you]", and B is " a thoroughly obstinate enemy of the prince". So the statement is: if someone keeps faith with you, then either they must be a very rare friend, or they must be a thoroughly obstinate enemy of the prince. Both of these possibilities are connected with keeping faith, by the grammatical structure of the sentence.

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In context, the person referred to as "you" here is not the prince but a conspirator against the prince:

A conspirator cannot act alone, nor can he take a companion except from those whom he believes to be dissatisfied. As soon as you have opened your mind to a dissatisfied person, you have given him the material with which to satisfy himself, because by informing on you he can look for every advantage. So, because of the certain gains from informing and the uncertain gains and certain dangers from conspiring, he must be a very true friend, or a thoroughly determined enemy of the prince, to keep faith with you.

The point of this passage is to justify the claim that conspiracies are rarely successful. To do this, Machiavelli puts himself in the shoes of a would-me conspirator ("you"), and imagines how such a person would find others to conspire with.