Ética, Parte III, Proposição 25

That, which we conceive to affect an object of our love pleasurably or painfully, affects us also pleasurably or painfully (III. xxi.). But the mind (III. xii.) endeavours, as far as possible, to conceive those things which affect us pleasurably; in other words (II. xvii. and Coroll.), it endeavours to regard them as present. And, contrariwise (III. xiii.), it endeavours to exclude the existence of such things as affect us painfully; therefore, we endeavour to affirm concerning ourselves, and concerning the loved object, whatever we conceive to affect ourselves, or the love object pleasurably. Q.E.D.