Metaphysics for Whitehead seems to be the final limit of speculative hypotheses that begin in particular fields of experience. (religion-online.org)
Klein's theoretical work gradually centered on a highly speculative hypothesis proposed by Sigmund Freud, namely that life is an anomaly — that it is drawn toward an inorganic state, and therefore, in an unspecified sense, contains an instinct to die. (newworldencyclopedia.org)
At this point the evidence remains circumstantial, and the whole concept is still a fairly speculative hypothesis. (discovermagazine.com)