The familiar phrase "the heart of the matter" comes pretty close to summing up the meaning of these words. Kernel , originating in an Old English word for corn seed, soon broadened to include any seed or cereal grain within its protective coating, then the edible inner part of a nut, and finally to its metaphorical sense as the central part of anything: The kernel of the problem is in the interpretation of the evidence.

Gist derives ultimately from a Latin word meaning to lie or rest. Its original meaning as a place of rest came to be extended to the ground or foundation on which something lay or by which it could be supported: the gist of an argument; the gist of the prosecution’s case against the accused. The closely related word substance (from the Latin for to stand under or beneath) includes among its numerous meanings the same idea of the essential or central part of anything, of that without which it would lack stability and value.

• Faith is the substance of things not seen; There is much substance in the critic’s views of modern art.

Crux , with its reference to the symbolism of the Cross, implies something pivotal, vital and, sometimes, as the word suggests, crucial: The closing of the gulf of Aqaba proved to be the crux of the situation. Nub is an Americanism meaning very much the same as gist , with an informal carry-over to the idea of a point or moral: the nub of the story; The students cannot go far wrong if they get the nub of the idea in their first lesson. Essence , from its scientific meaning of an extract by distillation, and thus the specific product in a reduced form, is very similar to gist and substance, but has some of the sense of necessity and vital nature of crux : The essence of his argument was clouded by the introduction of irrelevancies.

Nucleus , though it comes from the Latin word for nut, has not, like the related term kernel , acquired currency in a figurative sense. It has been, and still is, largely restricted to technical and scientific fields, where it denotes a central part or point round which other things are gathered, as in a cell, an atom, certain complex chemical compounds and the like. Because the ideas of movement, change and growth have become so closely associated with it, nucleus serves best only where such ideas are implied in the thought expressed: The nucleus of Plato’s philosophy was in his doctrine of the archetype.