How Concepts Become Possible: I. When an Idea Is in the Air

What does it mean for an idea to be possible before anyone has thought it?

We often tell the history of science as a succession of discoveries. Someone sees what others have missed, finds the solution to a problem, and changes our understanding of the world. The discovery is then attributed to the person who made it.

But this picture leaves something important unexplained.

Why do some discoveries appear independently to different people? Why do some ideas emerge almost simultaneously in different places? And why, once a particular idea has appeared, can it sometimes seem almost inevitable — even though nobody had thought of it before?

The history of natural selection offers a particularly revealing case.

Darwin and Wallace

Charles Darwin did not invent the idea of evolution. By the nineteenth century, the possibility that species might change over time was already being actively discussed. Geology had made the immense age of the Earth difficult to ignore. The fossil record revealed forms of life that no longer existed. Comparative anatomy and biogeography showed striking patterns of relationship and distribution.

There was also a growing recognition that organisms vary, and that breeders can deliberately select particular variations and reproduce them.

What was missing was an adequate account of how variation could become cumulative.

Darwin spent years developing such an account. Then, in 1858, he received an essay from Alfred Russel Wallace describing a remarkably similar mechanism of natural selection. The two men's work was presented jointly to the Linnaean Society.

The coincidence is important.

It does not mean that Darwin's achievement was inevitable. Nor does it diminish his originality. Darwin's On the Origin of Species was the product of an extraordinary synthesis of evidence, argument and conceptual development.

But the independent emergence of the same basic mechanism suggests that something had changed in the intellectual environment.

Natural selection had become possible in a new sense.

The possibility did not exist merely in Darwin's imagination. It existed in the relations among a growing number of observations, concepts and problems.

Possibility without inevitability

This is an important distinction.

To say that an idea is in the air is not to say that it must appear.

A possibility can be strongly afforded without being determined.

The evidence for biological variation did not logically entail natural selection. Malthus's argument about population pressure did not entail it. Artificial selection did not entail it. Geological gradualism did not entail it.

But these things could be brought into a relation in which natural selection became increasingly intelligible.

The possibility was therefore not simply a property of one person's mind. It belonged, at least partly, to the relational structure of the intellectual environment.

This gives us a different way of thinking about discovery.

Perhaps an idea does not first exist as a fully formed possibility and then get discovered. Perhaps the possibility itself emerges as relations among things change.

Before Darwin, the facts were already there. What was changing was the way those facts could constrain one another.

Variation could be related to inheritance.

Inheritance could be related to reproduction.

Reproduction could be related to population growth.

Population growth could be related to competition.

Competition could be related to differential survival and reproduction.

And suddenly a new possibility appeared:

selection could occur without a selector.

That last step is crucial. Artificial selection already provided a model in which variation could accumulate because an agent selected among variants. Darwin's conceptual achievement was to see that the same general process could occur without such an agent.

The environment could do the selecting.

The idea was not simply another fact added to the collection. It was a new relation among existing facts.

The difference a possibility makes

This also helps explain why the emergence of a concept can change the possibilities available afterward.

Once natural selection had been formulated, phenomena that previously appeared unrelated could be investigated in relation to it. Adaptation, mimicry, vestigial structures, geographical distribution and comparative anatomy could all become evidence within a new explanatory framework.

The concept therefore did not merely describe an existing field.

It reorganised the field.

This is one of the peculiar characteristics of conceptual evolution. A new concept can become an affordance for further concepts.

The possibility of natural selection made other biological questions possible in new ways. It opened a path through the intellectual landscape that had not previously been visible as such.

And yet that path was not created from nothing.

The landscape mattered.

The furrow and the plough

We might therefore distinguish two ideas that are easily confused.

An intellectual environment can determine a discovery.

Or it can make a discovery more probable.

The first would make conceptual history a kind of logical unfolding: given the preceding conditions, the discovery had to occur.

The second is more interesting.

The preceding conditions create affordances. They make some conceptual moves easier to make, some questions more salient, some solutions more intelligible and some connections more available than others.

But an affordance is not an instruction.

It does not tell the thinker what to think.

This may explain why independent discovery is so revealing. When two people arrive at substantially the same idea without collaborating, we have evidence that the intellectual environment was exerting a strong constraint on the space of possibilities.

But the very fact that they were separate thinkers reminds us that the environment did not itself produce the concept.

Something still had to happen.

Someone had to see the relation.

From possibility to actuality

This gives us a first distinction in the evolution of concepts.

Some conceptual possibilities are convergent.

They emerge when an intellectual environment has accumulated enough mutually reinforcing relations that several thinkers can independently arrive at substantially the same reconstruction.

Natural selection is a striking example.

But this is only one way in which concepts become possible.

Sometimes the evidence constrains a structure without uniquely determining it.

Sometimes an apparently impossible unification becomes possible when a new relation is discovered.

Sometimes the conceptual framework itself has to be reconceived.

And sometimes, as we shall see, a problem becomes unavoidable while the conceptual solutions proliferate in several incompatible directions.

The history of science is therefore not simply a history of people finding truths that were already waiting to be found.

It is also a history of possibility itself changing.

Natural selection gives us our first glimpse of this.

Darwin and Wallace did not merely discover the same idea.

They showed us that an idea can become in the air before it becomes anyone's possession.

The furrow can guide the plough.

But it does not determine exactly where the plough will go.