Species

Species and the Birth of Modern Science 53

In short, everything that can be, is, and the world is made to be everything it can

be. This is the source Leibniz’s doctrine of the lex completio that Voltaire so wick-

edly caricatured in his Candide as the teachings of Dr Pangloss. It is found in Plato’s

writings, but not in Aristotle, who famously wrote in the Metaphysics, “it is not nec-

essary that everything that is possible should exist in actuality,” and “it is possible for

that which has a potency not to realize it.”^21

However, the second plank of the Great Chain is the law of continuity (Leibniz

calls it the lex continui)—that all qualities must be continuous, not discrete. While

Aristotle did not make all things linear, arranged in a single ascending series, he

did require that there be no sudden “jumps,” from which the medieval claim natura

non facit saltus (nature does not make leaps) came, and which we shall meet again.

Aristotle’s version did not itself insist that one would classify a single living being in

one and only one series, nor that an organism that is graded as superior in one respect

must be superior in all, but that became the general impression later. Lovejoy says

It will be seen that there was an essential opposition between two aspects of Aristotle’s

inuence on subsequent thought, and especially upon the logical method not merely of

science but of everyday reasoning. ... He is oftenest regarded, I suppose, as the great

representative of a logic which rests upon the assumption of the possibility of clear

divisions and rigorous classication. Speaking of what he terms Aristotle’s “doctrine

of xed genera and indivisible species,” Mr. W. D. Ross has remarked that this was a

conclusion to which he was led mainly by his “close absorption in observed facts.” Not

only in biological species but in geometrical forms—... he had evidence of rigid clas-

sication in the nature of things. But this is only half of the story about Aristotle; and

it is questionable whether it is the more important half. For it is equally true that he

rst suggested the limitations and dangers of classication, and the non-conformity of

nature to those sharp divisions which are so indispensable for language and so conve-

nient for our ordinary mental operations. ...

From the Platonic principle of plenitude the principle of continuity could be

directly deduced. If there is between two given natural species a theoretically possible

intermediate type, that type must be realized—and so on ad indefinitum; otherwise

there would be gaps in the universe, the creation would not be as “full” as it might be,

and this would imply the inadmissible consequence that its Source or Author was not

“good,” in the sense which that adjective has in the Timaeus.^22

From Aristotle’s notion of an ontological scale, the higher beings were less poten-

tial and more determinate (God, the ens perfectissimum, could not be otherwise than

he is), “all individual things may be graded according to the degree to which they

are infected with [mere] potentiality.”^23 It is perhaps arguable if this idea really does

exist in Aristotle’s writings, or that he intended it; but whether or not he did, this

is the idea that was formulated by the Neo-Platonists, and various passages in his

works led to a serial classication of organisms.^24

(^21) Metaphysics II, 1003a 2, and XI, 1071b 13, quoted in Lovejoy 1936, 55.
(^22) Lovejoy 1936, 57f.
(^23) Ross, 1949, 178, after a discussion of the Metaphysics; quoted by Lovejoy 1936, 59.
(^24) Lovejoy 1936, 61–66 on Plotinus’ construction of the chain. Singer 1950, 40–41 on the Neo-Platonic
v iew.