Just look at it.

We could, of course, complicate the issue by a discussion of what constitutes 'green' - especially for those of us who are red-green colour blind!  But even they rarely have a problem agreeing the colour of grass. The point is that the "test" - to all normal intents and purposes - requires a very simple observation. The others, in contrast, require greater or lesser experimental effort in order to create opportunities for observations. They also - to a greater or lesser extent - need more sophisticated interpretation of any results.

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