The ability to concentrate attention – to be oblivious to distractions – is a sign of mind. Darwin, for instance, in his study of earthworms, noted that ‘Their sexual passion is strong enough to overcome for a time their dread of light,’ and took this as an indication of mental power:
When a worm is suddenly illuminated and dashes like a rabbit into its burrow – to use the expression employed by a friend – we are at first led to look at the action as a reflex one. … But the different effect which a light produced on different occasions, and especially the fact that a worm when in any way employed and in the intervals of such employment … is often regardless of light, are opposed to the view of the sudden withdrawal being a simple reflex action. With the higher animals, when close attention to some object leads to the disregard of the impressions which other objects must be producing on them, we attribute this to their attention being then absorbed; and attention implies the presence of a mind. Every sportsman knows that he can approach animals whilst they are grazing, fighting or courting, much more easily than at other times. The state, also, of the nervous system of the higher animals differs much at different times, for instance, a horse is much more readily startled at one time than at another. The comparison here implied between the actions of one of the higher animals and of one so low in the scale as an earth-worm, may appear far-fetched; for we thus attribute to the worm attention and some mental power, nevertheless I can see no reason to doubt the justice of the comparison.
— Darwin, The Formation of Vegetable Mould through the action of worms with observations of their habits (Project Gutenberg e-text vgmld10.txt)