Thus arose the celebrated “definitions,”—­the first step in Greek philosophy,—­intending to show what is, and what is not.  After demonstrating what is not, Socrates advanced to the demonstration of what is, and thus laid a foundation for certain knowledge:  thus he arrived at clear conceptions of justice, friendship, patriotism, courage, and other certitudes, on which truth is based.  He wanted only positive truth,—­something to build upon,—­like Bacon and all great inquirers.  Having reached the certain, he would apply it to all the relations of life, and to all kinds of knowledge.  Unless knowledge is certain, it is worthless,—­there is no foundation to build upon.  Uncertain or indefinite knowledge is no knowledge at all; it may be very pretty, or amusing, or ingenious, but no more valuable for philosophical research than poetry or dreams or speculations.

How far the “definitions” of Socrates led to the solution of the great problems of philosophy, in the hands of such dialecticians as Plato and Aristotle, I will not attempt to enter upon here; but this I think I am warranted in saying, that the main object and aim of Socrates, as a teacher of philosophy, were to establish certain elemental truths, concerning which there could be no dispute, and then to reason from them,—­since they were not mere assumptions, but certitudes, and certitudes also which appealed to human consciousness, and therefore could not be overthrown.  If I were teaching metaphysics, it would be necessary for me to make clear this method,—­the questions and definitions by which Socrates is thought to have laid the foundation of true knowledge, and therefore of all healthful advance in philosophy.  But for my present purpose I do not care so much what his method was as what his aim was.

The aim of Socrates, then, being to find out and teach what is definite and certain, as a foundation of knowledge,—­having cleared away the rubbish of ignorance,—­he attached very little importance to what is called physical science.  And no wonder, since science in his day was very imperfect.  There were not facts enough known on which to base sound inductions:  better, deductions from established principles.  What is deemed most certain in this age was the most uncertain of all knowledge in his day.  Scientific knowledge, truly speaking, there was none.  It was all speculation.  Democritus might resolve the material universe—­the earth, the sun, and the stars—­into combinations produced by the motion of atoms.  But whence the original atoms, and what force gave to them motion?  The proudest philosopher, speculating on the origin of the universe, is convicted of ignorance.