Abstract
"The first rule which the exact investigator of Nature should observe is, that he should not allow himself to pronounce an opinion, either in affirmation or denial, upon subjects which do not fall within the sphere of his observation or experience.....The second rule is, that he must not pass any opinion, form any judgment, nor utter it, upon matters of any science to the present level of which he has not brought himself." The words, which are Schleident's, occur in a tract, published at Leipsic in 1863, on the Materialism of the Recent German Scientific School (Ueber den Materialismus der Neueren Deutschen Natur-Wissenschaft). They are worth remembering when discussion the subject of this paper, in regard to which the controversy is not as to facts, but as tot he interpretation of facts. We wish also to bear in mind that to speculate where we cannot give proof is far easier than to believe where we cannot give proof is far easier than to believe where we cannot understand.