While it is true that science cannot decide questions of value, that is because they cannot be intellectually decided at all, and lie outside the realm of truth and falsehood. Whatever knowledge is attainable, must be attained by scientific methods; and what science cannot discover, mankind cannot know.
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Variants
"What science cannot tell us, mankind cannot know." (Attributed to Russell in Ted Peters' Cosmos As Creation: Theology and Science in Consonance [1989], p. 14, with a note that it was "told [to] a BBC audience [earlier this century]").