It is not clear exactly what Jesus means. Given the fact that he has been talking about where a person focuses his attention and energy, we can presume that his words have something to do with this. Perhaps he is emphasizing the result of a refusal to focus on God and his kingdom.
23 mais si ton œil est en mauvais état, tout ton corps sera dans les ténèbres. Si donc la lumière qui est en toi est ténèbres, combien seront grandes ces ténèbres!