One of the reasons the word why causes so much conceptual trouble is that it migrates easily across domains while quietly changing its function. What counts as an excellent answer to why in one domain becomes a category error in another.
Biology is a particularly instructive case. Here, why often appears to do real explanatory work — and, crucially, it does so without appealing to metaphysical purpose. This has led many readers, both sympathetic and hostile, to think that biology has somehow smuggled teleology back into science under a different name.
It hasn’t. But it has done something subtler, and more interesting.
The Appearance of Purpose
Consider the familiar form of biological explanation:
Birds have hollow bones because lighter skeletons aid flight.
To an unwary ear, this sounds goal-directed. The bones are hollow in order to make flight possible. Evolution, on this reading, looks like an engineer with foresight, selecting traits because of what they will later achieve.
Biologists are quick to deny this interpretation — rightly so. Natural selection does not anticipate outcomes. It does not plan. It does not aim.
But what, then, licenses the because?
Selection Is Not a Cause in the Ordinary Sense
Natural selection is often spoken of as a causal force, but this is already a metaphor — and a potentially misleading one. Selection does not push, pull, or produce traits the way a force produces motion or a chemical reaction produces a compound.
Instead, selection names a constraint structure:
given variation
given heredity
given differential survival and reproduction
certain trait distributions are internally necessary over time.
This is not causation layered on top of biology. It is biology articulating its own internal logic.
When we say that a trait exists because it confers a reproductive advantage, we are not identifying an efficient cause that brought the trait into being. We are identifying the conditions under which that trait is retained and stabilised within a population.
The because here is explanatory, not generative.
Internal Necessity, Not External Purpose
This is the same kind of necessity we saw in gauge symmetry, now playing out in a living system.
In physics, certain quantities must be conserved given the symmetry constraints of the theory.
In biology, certain traits must proliferate given the constraints of variation, heredity, and selection.
In neither case is there an external reason why the system itself exists or is the way it is. The explanations remain internal to the theoretical framework.
The mistake arises when this internal necessity is misread as an answer to a deeper ontological why:
Why do organisms exist at all?Why does life have this structure rather than another?
Natural selection does not answer these questions — and it does not pretend to. It answers a different one:
Given life of this kind, why do we observe these forms rather than others?
That is a perfectly legitimate because. It just isn’t the one people often think they’re hearing.
Why Biology Feels More Dangerous Than Physics
Biological explanations feel more philosophically loaded than physical ones because they track functional difference. Traits matter. They make a difference to survival and reproduction.
This difference easily slides into value-laden language:
better adapted
more efficient
successful strategies
But these are descriptions of system-relative success, not global purpose. The values are endogenous to the system, not imposed from outside it.
Confusing the two leads to familiar errors:
treating evolution as progressive
reading moral lessons into biology
assuming that explanatory success implies ontological completeness
Once again, the problem is not biology. It is the misread why.
Biology’s Proper Achievement
Biology shows us that because can be powerful without being metaphysical.
It can:
explain stability without intention
account for form without foresight
generate necessity without purpose
In doing so, it offers a model for how far explanation can go — and where it must stop.
The temptation to push further, to ask biology to tell us why there is life at all, is understandable. But it is a different question, requiring different tools.
In the next post, we will turn to a domain where the misread why becomes even more treacherous: social systems — where explanation, justification, and legitimacy are far harder to keep apart.
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