Since, about ten years ago, I again started to work at philosophy I have had to recognise grave mistakes in what I once set down in my book “Tractatus Logico-philosophicus”. What helped me to recognise these mistakes was – in a measure which I can hardly now estimate – the forceful criticism which my ideas received from F.P. Ramsey; with whom I went over them in innumerable discussions during the last two years of his life.– Even more, however, I owe to the criticism which Mr P. Sraffa, Lecturer in Economics at this University, has incessantly offered on my views. To this stimulus I owe the most fruitful of the thoughts I here communicate.