There is no ringing conclusion possible when we compare these types of thinking, with a view to telling which is the more absolutely true.  Their naturalness, their intellectual economy, their fruitfulness for practice, all start up as distinct tests of their veracity, and as a result we get confused.  Common sense is better for one sphere of life, science for another, philosophic criticism for a third; but whether either be truer absolutely, Heaven only knows.  Just now, if I understand the matter rightly, we are witnessing a curious reversion to the common-sense way of looking at physical nature, in the philosophy of science favored by such men as Mach, Ostwald and Duhem.  According to these teachers no hypothesis is truer than any other in the sense of being a more literal copy of reality.  They are all but ways of talking on our part, to be compared solely from the point of view of their use.  The only literally true thing is reality; and the only reality we know is, for these logicians, sensible reality, the flux of our sensations and emotions as they pass.  ‘Energy’ is the collective name (according to Ostwald) for the sensations just as they present themselves (the movement, heat, magnetic pull, or light, or whatever it may be) when they are measured in certain ways.  So measuring them, we are enabled to describe the correlated changes which they show us, in formulas matchless for their simplicity and fruitfulness for human use.  They are sovereign triumphs of economy in thought.

No one can fail to admire the ‘energetic’ philosophy.  But the hypersensible entities, the corpuscles and vibrations, hold their own with most physicists and chemists, in spite of its appeal.  It seems too economical to be all-sufficient.  Profusion, not economy, may after all be reality’s key-note.

I am dealing here with highly technical matters, hardly suitable for popular lecturing, and in which my own competence is small.  All the better for my conclusion, however, which at this point is this.  The whole notion of truth, which naturally and without reflexion we assume to mean the simple duplication by the mind of a ready-made and given reality, proves hard to understand clearly.  There is no simple test available for adjudicating offhand between the divers types of thought that claim to possess it.  Common sense, common science or corpuscular philosophy, ultra-critical science, or energetics, and critical or idealistic philosophy, all seem insufficiently true in some regard and leave some dissatisfaction.  It is evident that the conflict of these so widely differing systems obliges us to overhaul the very idea of truth, for at present we have no definite notion of what the word may mean.  I shall face that task in my next lecture, and will add but a few words, in finishing the present one.