Jay linked to this post regarding the beginning of human life. It has always amazed me that those who purport to use rational science and reasoning to support their position often do so in fallacious means. In these paragraphs in the middle of his post, he uses three different logical fallacies.
For one, any time someone suggests life "begins" you know that you're no longer talking about a biological problem but a moral or theological one. It is not a biological problem as life does not "begin" with each round of human reproduction. Life is continuous. Sperm are alive, and eggs are alive, and life has been a continuous stream of living organisms begetting more living organisms since it began in some form some 1 billion years ago. The appropriate question isn't whether life "begins" but rather when should we care?...Logical Fallacy: Equivocation
The phrase "life begins at conception" is basically an abbreviated version of "the life of an individual human organism begins at conception". It is true that life "began" a number of years ago, and no one in these bioethical debates questions that. The question is when an individual life begins. The author of the post must know this, but avoids the question by equivocating on the word "life".
But one could then argue, the fusion of the egg and a sperm is a "new" life. This isn't a great distinction either for a few reasons. For one, that would also make each egg and sperm that went through recombination and meiosis new life since they don't have a gene complement identical to their parent cells. So would cancer be "new" life. Just because something is new, doesn't create a valid argument, in my opinion, for its value or personhood.Logical Fallacy: Straw man argument
This one is very common. The author assumes that the pro-lifer believes that it is the mere presence of "new" DNA which confers value or personhood on an entity. The problem is that no pro-lifer that I know of makes this argument. The presence of "new" DNA is neither a sufficient (as the author points out) nor a necessary (in the case of twins) condition for a new human organism. He is taking out an argument that we do not make, and avoiding the one that we do.
Logical fallacy: Begging the Question
The best argument they have is that the embryo is a "potential" life - but is it really? Some 50% of fertilized eggs fail to implant, of those 50% that implant, the spontaneous miscarriage rate is about 10%. So 45% of the time fertilization might lead to a viable fetus. It is potential life, but there's already a great deal of waste of this kind of life that no one sheds a tear over - probably because they realize that it's not really a person being lost. Sperm and eggs then have potential of leading to a viable fetus too, what then makes the fertilized embryo more special? The higher probability of viability? What probability of forming a life then confers the value of personhood? Where is the threshold? Is wearing a condom then robbing sperm of their vital probability of making a new life equivalent to abortion? (some would say so - most would not)Either way, an embryo that hasn't been implanted doesn't really represent a new person, as it only has about a 45% potential of becoming life.
You may notice a straw man argument there also, but I'm sticking with the format. To beg the question is to assume something that you need to support with evidence. Here, the author simply assumes that an embryo that has not implanted is not "life" until it does implant. Not only does this assertion beg the question, it is demonstrably false.
I also wish to point out that he is using a horrendous argument. Lets say that someone is in a bad motor vehicle accident and he is given a 20% chance of surviving the night. He only has a 20% potential of being a live human organism in the morning. What does that fact say about his status at this time? Would anyone argue that the patient that they are working on is less than a human being because he does not have a great chance of not surviving? No way.