Now you will surely judge that a refusal of this kind to discuss matters publicly points to a high degree of inaccessibility to criticism, to obstinacy, or, in the polite colloquialism of the scientific world, to “pig-headedness.”[43] My reply to you would be that, should you have arrived at a conviction by means of such hard work, you would also thereby derive a certain right to maintain it with some tenacity. Further, on my own behalf, I can say that in the course of my work I have modified my views on important points, changed them or replaced them by others, and have of course in each case published the fact. What has been the result of this frankness? Some people have ignored my corrections of myself altogether and still to-day criticize me in respect of views which no longer mean the same to me. Others positively reproach me for these changes and declare me to be unreliable on that account. No one who changes his views once or twice deserves to be believed, for it is only too likely that he will be mistaken again in his latest assertions; but anyone who sticks to anything he has once said, or refuses to give way upon it easily enough, is obstinate or pig-headed; is it not so? What is to be done in the face of these self-contradictory criticisms except to remain as one is and behave as seems best to one? This is what I decided to do; and I am not deterred from remodelling and improving my theories in accordance with later experience. I have so far found nothing to alter in my fundamental standpoint and I hope this will never be necessary.