J.D. BARROW and F.J. TIPPLER formulated this principle (1986). According to them, the nature of the physical world must necessarily be such that the perceptive and intelligent observer's existence be possible.
A universe without witnesses would of course be a different universe… or would it "exist", i.e. is existence possible without self-consciousness (even of a part observing the whole – or at least a larger fragment of the whole)?
It is difficult to evaluate the possible systemic meanings of these recollections from PLATO and BERKELEY. Whiffs of solipsism are floating around!
A more acceptable view of the Anthropic Principle would be that the coherence of multiple characteristics of the universe as we know it (microphysic and macrophysic constants, space-time characteristics, gravitation, etc.) merely implies that our own existence as conscious observers (after a very long evolutive process!) became possible, or even normal.
But this kind of extraordinary stretched cosmical tautology does not lead to any new and significant rational development. It may be more useful for theology than for science. WALD, 1994, p. 123-31)