retrodict

retrodict \ret-ruh-DIKT\

Definition
verb
: to utilize present information or ideas to infer or explain (a past event or state of affairs)

Hindsight is 20-20? Not exactly. I really like MW's word of the day today. It means inferring backwards from the present state of things to explain the a past state of affairs we would otherwise have no or little information about - the origin of the universe, or the customs and beliefs of some ancient civilization.

While it's quite tempting to think of retrodiction as "predicting" the past, the reasoning employed isn't quite the same. Prediction is largely an inductive process. We see specific events happening, we develop theories about why (induction) and then project those theories into the future based on the current state of affairs and our theories. (Every time it has rained, the grass has gotten wet, therefore whenever it rains, the grass will be wet.) Retrodiction is largely abductive. It sees the present state of affairs and proposes an sufficient explanation. (The grass is wet, so it must have rained last night.) Of course, retrodiction is tethered to prediction. A theory which does not produce good predictions is unlikely to be employed in retrodiction. And thus we can "test" our retrodictions about past events.

To complete the picture, when we explain present events, we are using deduction.  It's raining now. The grass is wet. Therefore the rain made the grass wet. You'll note, of course, that there are other things which must also be true. There cannot be another source of moisture. There cannot be anything stopping the rain from reaching the grass. We make these and many other observations when we come to the simple deduction that we don't need to water the grass since it's raining.

So if science is largely a matter of induction, it is fair to say that it's mostly about prediction? I'd say yes. Even when science seeks to explain the past or present, it does so by creating theories that will predict the future. This is necessary for a theory to be testable.

The Truth, The Whole Truth, And Nothing But the Truth

"It can't be 'partly' true, it's either true or it's not!" he insisted.

"Well," I wryly replied, "that's partly true."

So what does "true" mean anyway? Can something be partially true and partially false? Or is a statement all true or false if it contains any part which is false? It's not so clear. For, in truth, the question of what is "true" is one of hotly contested debate for centuries that filled many volumes and is well beyond the scope of this site or post. Here's the Wikipedia entry on Truth.

What my friend meant was true in the Boolean sense. "A and B" is true if and only if "A" is true and "B" is true. So, in that sense, his statement was true.

I contend, in contrast, that outside the nice little world of pure logic, things get a lot stickier and it starts to make more sense to think about propositions as wholly or partly true. In fact, in real world examples, it starts to become harder to find propositions which are wholly and unambiguously true.

The stock market crash of 1929 caused the Great Depression is true, says I, but it's not an uncontested statement, if generally accepted. And it's not the whole story. Some would argue it's not even the primary cause. They'd argue that absent other factors, say, the reaction of the Federal Reserve, the recession might not have turned into a depression.

This leads to another observation, "partly" or "partially" true might mean that the statement is true in part and false in part or it might mean that it is only part of what is true. In everyday use, people use "partly true" to avoid rejecting whole cloth a statement which contains some truth but not entirely or to hedge particularly when they cannot dispute a statement as stated but feel that there are attenuating or mitigating factors that are of equal or greater importance that have not yet presented.