I think 'parsimony' and 'Occum's Razor' are the two concepts necessary to succinctly answer this question. To suggest that any biological organism (or any chemical or elementary particle for that matter) is intelligently designed is to introduce an assumption for which there is no support. But suggesting that humans or ETs designed all the crop circles is consistent with known phenomena (ie. intelligent biological organisms).
Parsimony and the Razor are never proofs of anything, but they are used to guide the formulation of a workable theory, and hence important concepts when deciding what and what not to regard as scientific.
With these two concepts alone we can confidently regard ID as not scientific, whilst cereology, in principle, is indeed scientific.
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