Nakamura, Hajime. 1975. Parallel Developments: A Comparative History of Ideas. Tokyo: Kodansha Ltd. (p. 225):
“The Buddha refused to give a definite answer on most metaphysical problems discussed in his day. Many ascetics were fond of asking questions concerning:
Whether the world is eternal or not,
Whether the world is infinite or not,
Whether the soul is the same as the body, or different from it,
Whether a saint exists in any way after death or not, and so on.
To all questions of this kind, the Buddha did not answer. Because he claimed it was not edifying nor connected with the essence of the norm (law), nor did it tend to the turning of the will, to the absence of passion, to cessation, to peace, to the higher faculties, to supreme wisdom, nor to nirvãna. 
The answering of all such questions, the Buddha held, would leave no time for finding the way to salvation or to liberation from suffering. He illustrated this by means of the following parable: A man is hit by a poisoned arrow. His friends hasten him to a doctor. The latter is about to draw the arrow from the wound. The wounded man, however, cries, ‘Stop! I will not have the arrow drawn out until I know who shot it, whether a warrior or a Brahmin, a common man (Vaiśya) or a slave (Śũdra), to which familiy he belonged, whether he was tall or short, or what kind the arrow was and its description’ , and so on. But the Buddha claimed that before all these questions were answered, the man would surely die. In the same way the disciple who wished for answers to all his questions about the beyond and such, would die before he knew the truth about suffering.”
 Majjhima-nikãya no. 63, pp. 431, 385. M. Winternitz, A History of Indian Literature, pp. 70-71. Rhys Davids, “Dialogues of the Buddha”, vol. I (Sacred Books of the Buddhists, vol. II, London, 1899), p. 186 f.
 H. Warren, Buddhism in Translations, pp. 117-128.