What does "the fitness of things" mean?

Question

I'm struggling to understand what it means, from this sentence:

In science, we sometimes have convictions which we cherish but cannot justify; we are influenced by some innate sense of the fitness of things.

Also, in:

A universe in which unprecedented and unpredictable events were at every moment flung into Nature would not merely be inconvenient to us: it would be profoundly repugnant. We will not accept such a universe on any terms whatever. It is utterly detestable to us. It shocks our 'sense of the fitness of things'.

(Context: Religion textbook)

Answer 1

The Cambridge online dictionary provides two definitions of fitness. The first is

The condition of being physically strong and healthy

and the second is

How suitable someone or something is

In the passages you quote it is, essentially, the second definition that is being used

Our sense of the fitness of things in this context means our sense of the appropriateness of the way inanimate objects and natural systems operate. That is we expect the world of inanimate objects to behave in a predictable fashion. For example we expect dense objects to fall, sparks from a fire to fly upwards an egg to break and not reassemble itself and so on. Seeing a dense object like a stone levitate spontaneously, sparks from a fire drop immediately to the ground or a broken egg suck its yolk and albumen into the broken halves of the shell bring the two halves and all the shards of shell together would disconcert us considerably because it would offend our sense of the way that we expect things to work and the way in which objects are affected by natural systems like gravity and entropy.

I suspect that this is either an old book (possibly a late 19th century one) or that the author is about to enter a discussion of the world of quantum mechanics in which branch of science things (usually at the subatomic level) do, indeed, behave in quite shocking ways.

Answer 2

Although discussion of the detailed application of the phrase involves opinion, there is a general meaning that is not dependent on opinion. I therefore do not vote to close, but instead attempt an answer.

One of the early examples of the use of the phrase is by William Paley:

Google
The fitness of things, means their fitness to produce happiness: the nature of things, means that actual constitution of the world, by which some things, as such and such actions, for example, produce happiness …

When we look for contemporary meaning, we find

Collins
Fitness:
1 the state of being fit
2 biology
a. the degree of adaptation of an organism to its environment, determined by its genetic constitution
b. the ability of an organism to produce viable offspring capable of surviving to the next generation

And

Cambridge
Fitness:
the quality of being suitable for someone or something

Without producing more tedious examples, it is clear that the term generally relates to some sense of order or purpose. The role of the world or the part of it in question is arranged such that it is ordered rather than chaotic or unpredictable, and such that some purpose is served, such as the pursuit of happiness, or the continuation and reproduction of things.

Answer 3

This should get you started:

fitness, n.
2. b. the (eternal) fitness of things: a phrase extensively used in the 18th cent. with reference to the ethical theory of Clarke, in which the quality of moral rightness is defined as consisting in a ‘fitness’ to the relations inherent in the nature of things. Hence popularly used (at first with playful allusion) for: What is fitting or appropriate.
_{Source: Oxford English Dictionary (login required)}