Science can never be more than an affirmation of certain things we believe in. These beliefs must be adopted responsibly, with due consideration of the evidence and with a view to universal validity. But eventually they are ultimate commitments, issued under the seal of our personal judgment. At some point we shall find ourselves with no other answer to queries than to say “because I believe so.” That is what no set of rules, or any model of science based on a system of rules, can do; it cannot say “because I believe so.” Only a person can believe something, and only I myself can hold my own beliefs. For the holding of these I must bear the ultimate responsibility; it is futile, and I think also ignoble, to hunt for systems and machines which will take that burden from 35 me. And we, as a community, must also face the fact that there is no system of necessary rules which will relieve us from the responsibility of holding the constitutive beliefs of our group or of teaching them to the next generation and defending their continued profession against those who would suppress them.