correct. it is from Aristotle, but if you are sure that a blastocyst is not an independent life form yet and that it becomes one at gastrulation than birth control in all forms is perfectly legitimate.  The rule used to be that the occurrence of twinning was enough to justify birth control - but the cardiologist who wrote Evangelicum Vitae ignored Church tradition and reasoning.  Mostly this branch of ethics has been dominated by papal fiat since Consortia Canubi, which is a perversion of natural law reasoning and must be rejected by thoughtful Catholics as implicit relativism.