Monday, October 10, 2011

Arguments from similarity

I thought some more about the evolution argument I discussed yesterday, and decided that, the argument is sufficiently general to be addressed by general logical principles, and does not require detailed knowledge of the science of DNA. The argument is that Unqualified appeals to similarity do not demonstrate common descent any more than they demonstrate common design. Which is absolutely correct. The argument “X’s are similar, therefore X’s have a common descent” lacks a middle, and is therefore invalid. To make it valid, we need a premiss that connects the term ‘similar’ to the term ‘have a common descent’. The premiss ‘X’s that are similar have a common descent’ would clearly do, but is clearly untrue. For example, pebbles that have been washed over by the tide for many years are all similar in that they are smooth, but they have no common descent. Snowflakes all display a common form, but have no common descent, etc.

The real principle we need to appeal to is that things which are similar are (or are highly likely to be) similar by a common nature or cause. The scholastics had a wonderful word for this: ratio, which has a common meaning that cannot be translated by any single word, but which in the present context would be translated by ‘reason’ or even ‘explanation’. When we see things with an identical or similar structure, there should be a explanation or reason for this. Pebbles washed by the tide are smooth for the very reason that they are washed by the tide, being knocked against other pebbles, smoothing the pointy bits. Snowflakes are similar because they are all water, which has a consistent molecular structure (at least I assume so, otherwise I’ll leave this to experts).

Primates and humans share similar DNA. There should be a reason for this. One reason is that God made the DNA similar. But is there a simpler explanation? Surely there is. Science tells us that DNA is a highly complex molecule. Common experience tells us that DNA is very ‘weak’ – it degrades quickly outside its natural environment, which is a living organism. So DNA cannot randomly occur. We also know, from common experience, that it can occur by reproduction. Humans reproduce other humans, primates reproduce primates, plants reproduce plants. (I have a spider plant in the attic that I have replicated for more than 20 years by the usual well-understood techniques). So, one simple explanation for the similarity of DNA is that it replicates itself. It reproduces its form in matter (Carbon, Hydrogen, Oxygen etc. molecules).

Thus, the simplest explanation of the similarity of form of DNA is by replication or reproduction. This does not require invoking a designer who caused the similarity.


David Brightly said...

I fail to see the point of the EvolutionNews article you link to, or indeed the WP page it locks horns with, except in so far as their respective authors see their efforts as propaganda. Both sides seem to write for an audience they think to be looking for some kind of proof of their respective positions. I may have a rather strict notion of proof but I can't see either side coming up with a convincing argument without adopting premisses even stronger and hence less acceptable than the ones they use now. 150-odd years ago Darwin looked around him and came up with a wonderful guess as to how the diversity of species arose. Since then we have been working out the details. The WP article is utterly ahistorical in that none of the observations it discusses could have played a part in Darwin's thinking. And yet none of them is, as far as I'm aware, inconsistent with the modern elaboration of his great idea. I'm content to leave it there.

Tony Lloyd said...

"But is there a simpler explanation?"

Putting your (adopted?) surname to one side, common descent is not just a simpler explanation but also a much more powerful one.

I think the creationist author used the term "unqualified" because it is only at the very simple, basic, unqualified level of "there are similarities" that "design" and "descent" are at all comparable.

The eyes of the fish, amphibians, reptiles and mammals are wired "backwards": the nerves are on the surface of the eye and the sensors that sense the light are covered by them. Mollusc eyes are not wired backwards, light enters the eye and falls directly onto the sensors, the nerve "wiring" is behind.

Common descent can explain that: the fish, mammals etc share a common ancestor with eyes whilst the common ancestor of that group and the molluscs was blind. Design cannot explain that, there is no reason a designer could not reuse the mollusc eye when coming up with frogs, or humans.

Richard said...

The principle to which you appeal, "The real principle we need to appeal to is that things which are similar are (or are highly likely to be) similar by a common nature or cause," needs justification. A weaker one, "things which are similar are (or are highly likely to be) similar by similar natures or causes," would seem sufficient.