Such was the assault on the feasability of Socrates's perfect society by Glaucon in Book V of Plato's Republic: in my interpretation, the pursuit of a perfect state is worthy even if only near-perfection is attainable.
Glaucon: But still I must say, Socrates, that if you are allowed to go on in this way you will entirely forget the other question which at the commencement of this discussion you thrust aside: --Is such an order of things possible, and how, if at all? For I am quite ready to acknowledge that the plan which you propose, if only feasible, would do all sorts of good to the State.
I will add, what you have omitted, that your citizens will be the bravest of warriors, and will never leave their ranks, for they will all know one another, and each will call the other father, brother, son; and if you suppose the women to join their armies, whether in the same rank or in the rear, either as a terror to the enemy, or as auxiliaries in case of need, I know that they will then be absolutely invincible; and there are many domestic advantages which might also be mentioned and which I also fully acknowledge: but, as I admit all these advantages and as many more as you please, if only this State of yours were to come into existence, we need say no more about them; assuming then the existence of the State, let us now turn to the question of possibility and ways and means --the rest may be left.
I have cut out some lines here for brevity but I highly recommend reading the entire work:
Socrates: Would a painter be any the worse because, after having delineated with consummate art an ideal of a perfectly beautiful man, he was unable to show that any such man could ever have existed?
Glaucon: He would be none the worse.
Socrates: Well, and were we not creating an ideal of a perfect State?
Glaucon: To be sure.
Socrates: And is our theory a worse theory because we are unable to prove the possibility of a city being ordered in the manner described?
Glaucon: Surely not.
Socrates: That is the truth. But if, at your request, I am to try and show how and under what conditions the possibility is highest, I must ask you, having this in view, to repeat your former admissions.
Glaucon: What admissions?
Socrates: I want to know whether ideals are ever fully realised in language? Does not the word express more than the fact, and must not the actual, whatever a man may think, always, in the nature of things, fall short of the truth? What do you say?
Glaucon: I agree.
Socrates: Then you must not insist on my proving that the actual State will in every respect coincide with the ideal: if we are only able to discover how a city may be governed nearly as we proposed, you will admit that we have discovered the possibility which you demand; and will be contented. I am sure that I should be contented --will not you?
Glaucon: Yes, I will.