In my second year of university I got elected as the president of the Pro-Life Club on campus. I had grown up being pro-life and had enjoyed arguing/discussing the issue, so I felt competent enough to run the club. However, in truth I had never done in-depth research on the topic, so being president for a year was a really great opportunity for me to critically evaluate different arguments from both sides. The results from that research weren’t always what I had expected; among other things, some of the most common pro-life arguments are pretty weak.
The SLED argument is probably the most commonly used Pro-Life argument to demonstrate moral equality between adults. SLED is an acronym for Size, Level of Development, Environment, and Dependency. This short list is meant to describe every major difference between humans of different ages and show that there is no meaningful difference. It usually goes something like this:
Look at an adult, a toddler, and a fetus. The toddler has a smaller size, is less developed, and is much more dependent on care from others than the adult, but the toddler is no less human and it’s just as wrong to murder it. Also, if the adult were to change environment, such as going into space, that does not change their humanity. Therefore, these differences are not enough to claim the unborn are not human. Since these are all of the differences, we must consider humans equal from the point of conception–or at least that’s the supposed conclusion.
This argument was developed by a fellow named Stephen Schwarz in the book The Moral Question of Abortion in 1990, and has been picked up by many major Pro-Life organizations since. The rhetorical strength of the argument is impressive. However, it has a pretty major logical failing.
The big issue is that every part of the argument besides environment is similar to The Paradox of The Heap. This is a classic philosophy question where there is a heap of many grains of sand and the challenge is to answer when it is no longer a heap if grains of sand are removed one at a time. 10,000 grains of sand might be considered a heap, just like 10,000 cells might be considered a human. 9,999 grains of sand may still be considered a heap, just like 9,999 cells may still be considered a human. But are 3 grains of sand a heap? Are 3 cells a human? The answer is a disappointing “maybe” unless we can define what exactly we mean by the words “heap” and “human.”
The real challenge is to define what a human is, and the word “human” here must mean an individual who is morally equivalent to an adult and should legally be considered a person. SLED is meant to show the unborn to be human in this way, which makes it a pretty big failure. It simply cannot accomplish this. It can’t even prove that a toddler is a human, it just relies on someone already accepting that as a fact. Then it tries to make people assume that because one degree of difference is not morally relevant, all degrees of difference must be morally irrelevant. That’s not much of an argument.
Many Pro-Life people just define a human as a legal and moral person from the moment of conception, which is certainly a possible route to take. If that definition can be demonstrated convincingly, though, there would be no need for the SLED argument. We could just point to the definition and measure humanity by that. However, a lot of folks have sincere and valid questions about that definition. If the Pro-Life cause has any hope, it’s in addressing those questions thoughtfully, not trying to SLED over them as quickly as possible.
So next time you hear someone trying to use the SLED argument ask them: are 3 grains of sand a heap? How about 2?
 Schlemon A. 2014 Mar 13. The S.L.E.D. Test. Stand to Reason. Signal Hill (CA): Stand to Reason APR; c2014 [accessed 2016 May 13]. http://www.str.org/articles/the-s.l.e.d.-test#.VzYBeb5FqpQ
 Wikipedia contributors. Sorites paradox [Internet]. Wikipedia, The Free Encyclopedia; 2016 Apr 16, 13:18 UTC [cited 2016 May 13]. Available from: https://en.wikipedia.org/wiki/Sorites_paradox