When one comes to understand these things, the history of philosophy no longer presents itself as a mere agglomeration of arbitrary and independent systems.  And an attentive reading gives us a further key to the interpretation of what seemed inexplicable.  We find that there may be distinct and different streams of thought, which, for a while, run parallel without commingling their waters.  For centuries the Epicurean followed his own tradition, and walked in the footsteps of his own master.  The Stoic was of sterner stuff, and he chose to travel another path.  To this day there are adherents of the old church philosophy, Neo-Scholastics, whose ways of thinking can only be understood when we have some knowledge of Aristotle and of his influence upon men during the Middle Ages.  We ourselves may be Kantians or Hegelians, and the man at our elbow may recognize as his spiritual father Comte or Spencer.

It does not follow that, because one system follows another in chronological order, it is its lineal descendant.  But some ancestor a system always has, and if we have the requisite learning and ingenuity, we need not find it impossible to explain why this thinker or that was influenced to give his thought the peculiar turn that characterizes it.  Sometimes many influences have conspired to attain the result, and it is no small pleasure to address oneself to the task of disentangling the threads which enter into the fabric.

Moreover, as we read thus with discrimination, we begin to see that the great men of the past have not spoken without appearing to have sufficient reason for their utterances in the light of the times in which they lived.  We may make it a rule that, when they seem to be speaking arbitrarily, to be laying before us reasonings that are not reasonings, dogmas for which no excuse seems to be offered, the fault lies in our lack of comprehension.  Until we can understand how a man, living in a certain century, and breathing a certain moral and intellectual atmosphere, could have said what he did, we should assume that we have read his words, but not his real thought.  For the latter there is always a psychological, if not a logical, justification.

And this brings me to the question of the language in which the philosophers have expressed their thoughts.  The more attentively one reads the history of philosophy, the clearer it becomes that the number of problems with which the philosophers have occupied themselves is not overwhelmingly great.  If each philosophy which confronts us seems to us quite new and strange, it is because we have not arrived at the stage at which it is possible for us to recognize old friends with new faces.  The same old problems, the problems which must ever present themselves to reflective thought, recur again and again.  The form is more or less changed, and the answers which are given to them are not, of course, always the same.  Each age expresses itself in a somewhat different way.  But sometimes the solution proposed for a given problem is almost the same in substance, even when the two thinkers we are contrasting belong to centuries which lie far apart.  In this case, only our own inability to strip off the husk and reach the fruit itself prevents us from seeing that we have before us nothing really new.