Published on June 23, 2025 9:07 PM GMT
Salutations,
I have been a regular reader (and big fan) of LessWrong for quite some time now, so let me just say that I feel honoured to be able to share some of my thoughts with the likes of you folks.
I don't reckon myself a good writer, nor a very polished thinker (as many of the veteran writers here), so I hope you'll bear with me and be gentle with your feedback (it is my first time after all).
Without further ado, I have been recently wrestling with the concept of abductive reasoning. I have been perusing for good definitions and explanations of it, but none persuade me that abductive reasoning is actually a needed concept.
The argument goes as follows: “Any proposed instance of abductive reasoning can be fully explained by a combination of inductive and deductive reasoning. Therefore, abductive reasoning is unnecessary — aka it does not « exist » as a form of reasoning.”
Now, the idea that a form of reasoning that has been accepted wisdom for over a century might be bogus sounds a bit far-fetched. (That much more so because I very much respect the work of Charles Sanders Peirce, who first proposed the concept). I am sure that I am missing something, but for the love of logic, it’s not coming to me. So I am hoping that some clever comment might help me figure it out.
But first, here’s a more detailed explanation of my argument.
Deductive reasoning can be defined as constructing formal logical arguments, such that if the premises are true, then the conclusion must be true.
Example: “Humans are fragile. I am a human. Therefore, I am fragile.”
Inductive reasoning is about generalising a body of observations to formulate general principles.
Example: “For the past 7 days it has been raining. Therefore, tomorrow it will probably also rain.”
It is often said that: "The conclusions of deductive reasoning are certain, whereas those of inductive reasoning are probably". I think this contrast is somewhat misleading and imprecise, as the certainty of deductive conclusions just means that they necessarily follow from the premises (they are implied by the premises), but the conclusion itself might still be probabilistic.
Example: "If I have a fever, there’s a 65% probability that I have the flu. I have a fever. Therefore, there’s a 65% probability that I have the flu."
Finally, abductive reasoning (also known as “inference to the best explanation”) is about finding the most likely explanation for one or few observations.
Example: “I have a fever, cough and I am feeling weak. Therefore, I most likely have the flu”.
Now let’s attack the example of abductive reasoning, starting with the conclusion “I most likely have the flu”.
This is logically equivalent to “Among the potential explanations for my symptoms that I could come up with, having the flu has the highest probability of being true”.
But where are the probabilities coming from? They must come from inductive reasoning (looking at past instances of symptoms and diseases) or deductive reasoning (e.g., from your existing knowledge).
Note that we don’t really care if the probabilities are right. We also don’t care if I am examining all the possible diseases or just a handful that came to mind (and maybe none of them is right).
The point is that you can reconstruct the argument as:
- Inductive reasoning: Given past data, the probability of having a given disease given symptom X are [...]Deductive reasoning: the probabilities are [...] → I have symptom X → I have disease Y with probability > 50% (Deductive reasoning) == “I most likely have disease Y”
There’s no “leap of faith”, no “abduction” happening. Just inducting probabilities and then deducting the most likely outcome.
What if there is no prior data about this event to use to induce probabilities?
It doesn't matter; you can still deduce your way to probability estimates from other beliefs, analogous events, and adjacent knowledge. (Curiously, I haven't seen a LessWrong article on how to correctly make probability estimates in the absence of direct data, can anyone point me to it?)
The point is that you must arrive at some probability distribution of explanations to be able to declare one as the "most likely" (which is in the definition of abductive reasoning).
If you had absolutely no prior knowledge (e.g., you were dropped in a completely new universe), then the rational stance would be to consider all outcomes as equally likely - which means none can be considered the most likely.
In conclusion, to paraphrase Eliezer, abductive reasoning is starting to sound a lot like "magical reasoning". It's unclear what it actually means, and I suspect it just came to represent a mix of subconscious search over the hypothesis space and estimation of the probability distribution of possible explanations. But just because some reasoning is subconscious, it doesn't make it a new form of reasoning.
But as I said, simply based on the historical endurance of this term, I’m probably missing something here.
Discuss
