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From ScienceDaily:

From a big-picture perspective, if we didn't possess this powerful oculomotor mechanism, particularly when blinking, our surroundings would appear shadowy, erratic and jittery, researchers said.

Why is it not like this:

From a big-picture perspective, if we had not possessed this powerful oculomotor mechanism, particularly when blinking, our surroundings would have appeared shadowy, erratic and jittery, researchers said.

After all, we do possess this mechanism, so we should use the had not possessed / would have appeared to indicate the counterfactual nature of the statement, shouldn't we?

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The so-called 'past' forms did and would are employed to express present counterfactuality. The corresponding real situation is expressed with 'present' forms:

If we do not possess this mechanism our surroundings will appear shadowy.

The constructions with HAVE + past participle (I call this the 'sham perfect') are used to express past counterfactuality:

If we had not possessed this mechanism (long ago), our surroundings would have appeared shadowy (at that time).

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