All such illusions are immediately present to us as perceptions, and cannot be dispelled by any arguments of the reason. Reason can only prevent error, that is, a judgment on insufficient grounds, by opposing to it a truth; as for example, the abstract knowledge that the cause of the weaker light of the moon and the stars at the horizon is not greater distance, but the denser atmosphere; but in all the cases we have referred to, the illusion remains in spite of every abstract explanation. For the understanding is in itself, even in the case of man, irrational, and is completely and sharply distinguished from the reason, which is a faculty of knowledge that belongs to man alone. The reason can only _know_; perception remains free from its influence and belongs to the understanding alone. § 7. With reference to our exposition up to this point, it must be observed that we did not start either from the object or the subject, but from the idea, which contains and presupposes them both; for the antithesis of object and subject is its primary, universal and essential form. We have therefore first considered this form as such; then (though in this respect reference has for the most part been made to the introductory essay) the subordinate forms of time, space and causality. The latter belong exclusively to the _object_, and yet, as they are essential to the object _as such_, and as the object again is essential to the subject _as such_, they may be discovered from the subject, _i.e._, they may be known _a priori_, and so far they are to be regarded as the common limits of both. But all these forms may be referred to one general expression, the principle of sufficient reason, as we have explained in the introductory essay.