Where does [evil] come from, then? The only way to get a bad thing is to take a good thing and spoil it. For example, darkness isn’t made up from nothing; you get it by blocking the light. Disease isn’t made up from nothing; you get it by ruining health. Notice that this doesn’t work the other way around – you can’t get light by blocking darkness or health by ruining disease. So God created only good things, but some of them have been spoiled.