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I am very pleased with this suggestion, but for all that I cannot

regard the matter as quite settled, because I feel constrained to

raise the following objection: "Your definition would certainly

be right, if only I knew that the light by means of which the

observer at M perceives the lightning flashes travels along the

length A → M with the same velocity as along the length B → M.

But an examination of this supposition would only be possible if

we already had at our disposal the means of measuring time. It

would thus appear as though we were moving here in a logical

circle."