What Is a First Principle? Reasoning From the Bottom Rather Than the Side
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Reasoning from first principles means stripping a question back to what is actually known to be true and building up from there, rather than reasoning by analogy to how things are usually done. The idea is ancient, the modern business usage is looser, and both are useful.
The philosophical origin
Aristotle used the term for the basic propositions of a field, which cannot themselves be derived from anything more basic within it and which everything else in the field rests on. That raises the immediate question of how such principles are known, since they cannot be proved by the method they underwrite, and his answer involved a form of intuitive grasp arrived at through experience. Descartes took the project further by attempting to demolish everything he could doubt and rebuild from whatever survived, arriving at his own thinking as the one thing that could not be doubted while doubting, and then attempting to reconstruct knowledge from there. That programme is generally regarded as having failed in its own terms, since the reconstruction relies on arguments most philosophers do not accept, and the attempt established a great deal about the shape of the problem. The underlying question of what justification rests on, and whether it must stop somewhere, remains open and connects directly to the modern debates about foundations.
The practical version
The engineering and business use is looser and genuinely productive when applied properly:
- •Identify the assumptions embedded in how something is currently done, which is the hardest step since they are invisible precisely because everyone shares them
- •Ask which of those are physical or logical constraints and which are conventions, historical accidents or artefacts of somebody else's circumstances
- •Reduce the problem to the constraints that genuinely cannot be moved, typically physics, arithmetic and law
- •Cost or evaluate the solution from those constraints upward, which is the step that reveals whether current practice is near the achievable limit or far from it
- •Rebuild a solution respecting only the real constraints
- •The publicised example is the cost of a battery pack computed from the market prices of its constituent materials, which produced a figure far below the prevailing pack price and motivated an attempt to close the gap
Why it works and when it does not
The method works because conventions accumulate, and a practice established under conditions that no longer hold persists because nobody revisits it. Reasoning by analogy, which is the default, propagates those conventions along with the genuine constraints, so an industry can settle at a level of cost or performance far from what is possible without anyone being wrong at any particular step. Going back to the constraints identifies the gap. It fails in identifiable ways. It is expensive, since working out from scratch what everyone else already knows wastes effort on the many occasions when convention is correct. It underrates tacit knowledge, since the reasons for a practice are frequently real and undocumented, and discovering them by rediscovery is costly and occasionally dangerous. It is vulnerable to the practitioner missing a constraint they do not know about, which produces a confident plan that fails on contact. And it can become a rhetorical posture, used to dismiss expertise rather than to examine assumptions, which is the commonest failure in its popular form.
Where it is standard practice
The approach is routine in fields that do not call it by that name. Physics estimates from fundamental quantities before building anything, which is the same operation. Engineering computes theoretical limits, including the maximum efficiency of an engine or the minimum energy for a separation process, and compares real performance against them, which immediately indicates whether improvement is available. Cost accounting uses should-cost modelling, building up a product's cost from materials, labour and process rather than from what suppliers quote, which is a standard procurement technique. Scientific research designs from the question rather than from the available method, at least in principle. Legal reasoning goes to the statute and the constitutional text when precedent is unsatisfactory. The common thread is having an independent standard against which current practice can be judged, and the value lies less in inventing new solutions than in knowing whether the existing one is any good.
The takeaway
Stripping a question to the constraints that genuinely cannot move, then building upward, catches conventions that persist because nobody revisits them, which reasoning by analogy propagates unexamined. The approach is expensive, underrates undocumented reasons for existing practice, and fails when a constraint is missed. Its real value is supplying an independent standard for judging whether current practice is anywhere near what is achievable.