Some things are real and some are not; as a rule he seems to be able to discover the difference; of his method of procedure he has never tried to give an account to himself.
That he has a mind he cannot doubt, and he has some idea of the difference between it and certain other minds; but even the most ardent champion of the plain man must admit that he has the most hazy of notions touching the nature of his mind.

He seems to be more doubtful concerning the nature of the mind and its knowledge than he is concerning the nature of external things.

Certainly he appears to be more willing to admit his ignorance in this realm.
And yet the man can hold his own in the world of real things.

He can distinguish between this thing and that, this place and that, this time and that.

He can think out a plan and carry it into execution; he can guess at the contents of other minds and allow this knowledge to find its place in his plan.
All of which proves that our knowledge is not necessarily useless because it is rather dim and vague.