How do you recognise common objects?
Consider how you identify your close relatives or friends. Most people will recognise them without consciously thinking to themselves how they are doing it. If, on the other hand you had to write a field guide for others to identify your friends and family, you would have to start analysing, maybe for the first time, what features a stranger might be able to pick on to identify them.
Experts, whether scientists or farmers, tend to subconsciously recognise their plants, apparently without a moment’s thought. Learners, or experts when confronted by a plant at the limits of their expertise, will tend think more analytically (‘It can’t be species A, because it’s in a swamp, so it must be species B’). Even the most academic and analytically-minded expert will probably suspect that a species is a new one simply by realising that they do not recognise it. That opinion will then be confirmed or refuted in the scientific case by working slowly and systematically through maybe hundreds of individual descriptions to rule out existing names. The only option for non scientists in the same situation is to find someone else who does know the plant, or make a name up for personal use.
It is a common thought that the most useful approach to uncovering secrets of plant recognition would be to ask a local expert “what is the name of that plant” and then “how do you recognise it”. This sounds straightforward, but can be a frustrating approach if, as is normal, experts are relying largely on recognition. Of course, it is very useful to involve local experts, or other experts if you are a local expert, to ask what they think the plant names are, and why. Probably, you will learn something useful for your field guide. But, often it is apparent that the expert does not know entirely on what basis they are recognising the plants they know. The secretly bemused expert may, when asked, latch onto any obvious feature –maybe the grey bark - struggling to translate recognition into analysis. When similar bark is later seen on another species the new rule sprouts complex sub-clauses, ifs and buts, yet all the time the expert was not really using these diagnostic rules anyway.
Psychologists say that a large part of how we recognise things is based on some intangible ‘gestalt’, or ‘spirit of the whole’, rather than the results of some speedy analysis of successive questions and answers and, on computers, ‘neural networks’ can be trained to behave in a similar way, However, even when a computer neural network has been trained reliably to recognise a spanner on a conveyor belt as distinct from a hammer, it is not then possible to open up the circuit and obtain a simple set of rules about hammer vs. spanner which would be any diagnostic use elsewhere. Do not be surprised when the expert’s brain is found to contain no speakable rules for separating two species which that same expert can, however, reliably distinguish in practice.
And do not be surprised if, after years of familiarity, you can distinguish two species, but find it hard to write a guide that explains how. Unless, of course, you can remember the diagnostic questions from a field guide or expert by which you first learnt to distinguish them.