THREE:To believe a fact is not to learn it, in the sense that these terms may be applied to mechanical knowledge; to believe a proposition is not to have a conviction of its truth; and what is meant by learning mechanical principles is, as remarked in a previous place, to have them so fixed in the mind that they will involuntarily arise to qualify everything met with that involves mechanical movement. For this reason it has been urged that learners should begin by first acquiring a clear and fixed conception of power, and next of the nature and classification of machines, for without the first he cannot reach the second.Our survey of Platos first period is now complete; and we have to enter on the far more arduous task of tracing out the circumstances, impulses, and ideas by which all the scattered materials of Greek life, Greek art, and Greek thought were shaped into a new system and stamped with the impress of an imperishable genius. At the threshold of this second period the personality of Plato himself emerges into greater distinctness, and we have to consider what part it played in an evolution where universal tendencies and individual leanings were inseparably combined.