A human life is simply any cell of human nature. One cell kept alive in a laboratory could be considered human life, but it is still obviously not a human being. A human being is a, "biologically whole individual of the species" (Kreeft,5) So as of now even the pro-choice people have defined the fetus as having human life, and as a human being. Now is the seemingly controversial point, is the fetus a human person? In favour of abortion one would say that because the fetus is part of a whole person (the mother) than the fetus itself can not be its own person and not until it is separated from the mother is the fetus a human person. Also a pro-choicer would say that a zygote does not have the ability to think, chose, or love or any of the other actions which make a human distinctively a human. This however can be refuted quite easily by observing the fact that our personhood does not merely come to us in an instant but instead developed over time. This is seen throughout a persons whole life. A child is not the same person when he becomes a teenager and in the same way this same teenager is not the same person as he will be when he is a fully grown adult. So who is to say that the personhood of the person at hand is not merely in its early stages of development If this were to be the case then the abrupt begin of personhood would be conception, which is really the only logical conclusion seeing as how otherwise there would be no absolute standard for the beginning of personhood for all people. Because all humans share the moment of conception without any exceptions, it must be the beginning of personhood.
The most fundamental argument for pro lifers, in a sense that it is so basic and logical that it can really not be refuted. Kreeft's begins with a statement that absolutely noncontroversial and builds from there. The first premise to his argument is that we know what an apple is. To say that we know what an apple is can not be contested and if one were to try, he would be seen as a fool.