Let us endeavour to do our best: beware of the poisonous reptiles--that is to say, the bad thoughts and aridities which are often permitted by God to assail and torment us so that we cannot repel them. Indeed, perchance we feel their sting! He allows this to teach us to be more on our guard in the future and to see whether we grieve much at offending Him. Therefore if you occasionally lapse into sin, do not lose heart and cease trying to advance, for God will draw good even out of our falls, like the merchant who sells theriac, who first takes poison, then the theriac, to prove the power of his elixir. This combat would suffice to teach us to amend our habits if we realized our failings in no other way, and would show us the injury we receive from a life of dissipation. Can any evil be greater than that we find at home? What peace can we hope to find elsewhere, if we have none within us? What friends or kindred can be so close and intimate as the powers of our soul, which, whether we will or no, must ever bear us company? These seem to wage war on us as if they knew the harm our vices had wrought them. 'Peace, peace be unto you,' my sisters, as our Lord said, and many a time proclaimed to His Apostles. Believe me, if we neither possess nor strive to obtain this peace at home, we shall never find it abroad.
St. Teresa of Avila, Interior Castle, Second Mansions