From the Visuddhimagga Part III, Ch XVI:
"[DISCUSSION ON NIBBÁNA]
67. [Question 1] Is Nibbána non-existent because it is unapprehendable, like
the hare’s horn?
[Answer] That is not so, because it is apprehendable by the [right] means. For
it is apprehendable [by some, namely, the nobles ones] by the [right] means, in
other words, by the way that is appropriate to it, [the way of virtue, concentration,
and understanding]; it is like the supramundane consciousness of others, [which
is apprehendable only by certain of the Noble Ones] by means of knowledge of
penetration of others’ minds. Therefore it should not be said that it is non-existent
because unapprehendable; for it should not be said that what the foolish ordinary
man does not apprehend is unapprehendable.
68. Again, it should not be said that Nibbána does not exist. Why not? Because
it then follows that the way would be futile.  For if Nibbána were nonexistent,
then it would follow that the right way, which includes the three
aggregates beginning with virtue and is headed by right understanding, would
be futile. And it is not futile because it does reach Nibbána."
I find the whole discussion on nibbana in this section quite interesting.