The 100 Most Influential Scientists of All Time (4 page)

Aristotle's vision of the cosmos also owes much to Plato's dialogue
Timaeus
. As in that work, the Earth is at the centre of the universe, and around it the Moon, the Sun, and the other planets revolve in a succession of concentric crystalline spheres. The heavenly bodies are not compounds of the four terrestrial elements but are made up of a superior fifth element, or “quintessence.” In addition, the heavenly bodies have souls, or supernatural intellects, which guide them in their travels through the cosmos.

Even the best of Aristotle's scientific work has now only a historical interest. The abiding value of treatises such as the
Physics
lies not in their particular scientific assertions but in their philosophical analyses of some of the concepts that pervade the physics of different eras—concepts such as place, time, causation, and determinism.

In his
Posterior Analytics
, Aristotle applies the theory of the syllogism (a form of deductive reasoning) to scientific and epistemological ends (epistemology is the philosophy of the nature of knowledge). Scientific knowledge, he urges, must be built up out of demonstrations. A demonstration is a particular kind of syllogism, one whose premises can be traced back to principles that are true, necessary, universal, and immediately intuited. These first, self-evident principles are related to the conclusions of science as axioms are related to theorems: the axioms both necessitate and explain the truths that constitute a science. The most important axioms, Aristotle thought, would be those that
define the proper subject matter of a science. Thus, among the axioms of geometry would be the definition of a triangle. For this reason much of the second book of the
Posterior Analytics
is devoted to definition.

The account of science in the
Posterior Analytics
is impressive, but it bears no resemblance to any of Aristotle's own scientific works. Generations of scholars have tried in vain to find in his writings a single instance of a demonstrative syllogism. Moreover, the whole history of scientific endeavour contains no perfect instance of a demonstrative science.

(b. 23 CE, Novum Comum, Transpadane Gaul [now in Italy]—d. Aug. 24, 79, Stabiae, near Mt. Vesuvius)

P
liny the Elder (Latin: Gaius Plinius Secundus) was a Roman savant and author of the celebrated
Natural History
, an encyclopaedic work of uneven accuracy that was an authority on scientific matters up to the Middle Ages. Seven writings are ascribed to Pliny, of which only the
Natural History
is extant. There survive, however, a few fragments of his earlier writings on grammar, a biography of Pomponius Secundus, a history of Rome, a study of the Roman campaigns in Germany, and a book on hurling the lance. These writings probably were lost in antiquity and have played no role in perpetuating Pliny's fame, which rests solely on the
Natural History
.

The
Natural History
, divided into 37
libri
, or “books,” was completed, except for finishing touches, in 77 CE. In the preface, dedicated to Titus (who became emperor shortly before Pliny's death), Pliny justified the title and explained his purpose on utilitarian grounds as the study of “the nature of things, that is, life.” Heretofore, he continued, no one had attempted to bring together the older,
scattered material that belonged to “encyclic culture” (
enkyklios paideia
, the origin of the word encyclopaedia). Disdaining high literary style and political mythology, Pliny adopted a plain style—but one with an unusually rich vocabulary—as best suited to his purpose. A novel feature of the
Natural History
is the care taken by Pliny in naming his sources, more than 100 of which are mentioned. Book I, in fact, is a summary of the remaining 36 books, listing the authors and sometimes the titles of the books (many of which are now lost) from which Pliny derived his material.

The
Natural History
properly begins with Book II, which is devoted to cosmology and astronomy. Here, as elsewhere, Pliny demonstrated the extent of his reading, especially of Greek texts. By the same token, however, he was sometimes careless in translating details, with the result that he distorted the meaning of many technical and mathematical passages. In Books III through VI, on the physical and historical geography of the ancient world, he gave much attention to major cities, some of which no longer exist.

Books VII through XI treat zoology, beginning with humans, then mammals and reptiles, fishes and other marine animals, birds, and insects. Pliny derived most of the biological data from Aristotle, while his own contributions were concerned with legendary animals and unsupported folklore.

In Books XII through XIX, on botany, Pliny came closest to making a genuine contribution to science. Although he drew heavily upon Theophrastus, he reported some independent observations, particularly those made during his travels in Germany. Pliny is one of the chief sources of modern knowledge of Roman gardens, early botanical writings, and the introduction into Italy of new horticultural and agricultural species. Book XVIII, on
agriculture, is especially important for agricultural techniques such as crop rotation, farm management, and the names of legumes and other crop plants. His description of an ox-driven grain harvester in Gaul, long regarded by scholars as imaginary, was confirmed by the discovery in southern Belgium in 1958 of a 2nd-century stone relief depicting such an implement. Moreover, by recording the Latin synonyms of Greek plant names, he made most of the plants mentioned in earlier Greek writings identifiable.

Books XX through XXXII focus on medicine and drugs. Like many Romans, Pliny criticized luxury on moral and medical grounds. His random comments on diet and on the commercial sources and prices of the ingredients of costly drugs provide valuable evidence relevant to contemporary Roman life. The subjects of Books XXXIII through XXXVII include minerals, precious stones, and metals, especially those used by Roman craftsmen. In describing their uses, he referred to famous artists and their creations and to Roman architectural styles and technology.

Perhaps the most important of the pseudoscientific methods advocated by Pliny was the doctrine of signatures: a resemblance between the external appearance of a plant, animal, or mineral and the outward symptoms of a disease was thought to indicate the therapeutic usefulness of the plant. With the decline of the ancient world and the loss of the Greek texts on which Pliny had so heavily depended, the
Natural History
became a substitute for a general education. In the European Middle Ages many of the larger monastic libraries possessed copies of the work. These and many abridged versions ensured Pliny's place in European literature. His authority was
unchallenged, partly because of a lack of more reliable information and partly because his assertions were not and, in many cases, could not be tested.

However, Pliny's influence diminished starting in the late 15th century, when writers began to question his statements. By the end of the 17th century, the
Natural History
had been rejected by the leading scientists. Up to that time, however, Pliny's influence, especially on nonscientific writers, was undiminished. He was, for example, almost certainly known to William Shakespeare and John Milton. Although Pliny's work was never again accepted as an authority in science, 19th-century Latin scholars conclusively demonstrated the historical importance of the
Natural History
as one of the greatest literary monuments of classical antiquity.

(b.
c
. 100 CE—d.
c
. 170)

P
tolemy (Latin: Claudius Ptolemaeus) was an Egyptian astronomer, mathematician, and geographer of Greek descent who flourished in Alexandria during the 2nd century CE. In several fields his writings represent the culminating achievement of Greco-Roman science, particularly his geocentric (Earth-centred) model of the universe now known as the Ptolemaic system.

Virtually nothing is known about Ptolemy's life except what can be inferred from his writings. His first major astronomical work, the
Almagest
, was completed about 150 CE and contains reports of astronomical observations that Ptolemy had made over the preceding quarter of a century. The size and content of his subsequent literary production suggests that he lived until about 170 CE.

The book that is now generally known as the
Almagest
(from a hybrid of Arabic and Greek, “the greatest”) was
called by Ptolemy
Hē mathēmatikē syntaxis (The Mathematical Collection
) because he believed that its subject, the motions of the heavenly bodies, could be explained in mathematical terms. The opening chapters present empirical arguments for the basic cosmological framework within which Ptolemy worked. Earth, he argued, is a stationary sphere at the centre of a vastly larger celestial sphere that revolves at a perfectly uniform rate around Earth, carrying with it the stars, planets, Sun, and Moon—thereby causing their daily risings and settings. Through the course of a year the Sun slowly traces out a great circle, known as the ecliptic, against the rotation of the celestial sphere. The Moon and planets similarly travel backward against the “fixed stars” found in the ecliptic. Hence, the planets were also known as “wandering stars.” The fundamental assumption of the
Almagest
is that the apparently irregular movements of the heavenly bodies are in reality combinations of regular, uniform, circular motions.

How much of the
Almagest
is original is difficult to determine because almost all of the preceding technical astronomical literature is now lost. Ptolemy credited Hipparchus (mid-2nd century BCE) with essential elements of his solar theory, as well as parts of his lunar theory, while denying that Hipparchus constructed planetary models. Ptolemy made only a few vague and disparaging remarks regarding theoretical work over the intervening three centuries; yet the study of the planets undoubtedly made great strides during that interval. Moreover, Ptolemy's veracity, especially as an observer, has been controversial since the time of the astronomer Tycho Brahe (1546–1601). Brahe pointed out that solar observations Ptolemy claimed to have made in 141 BCE are definitely not genuine, and there are strong arguments for doubting that Ptolemy independently observed the more than 1,000 stars listed in his star catalog. What is not disputed, however, is the mastery of mathematical analysis that Ptolemy exhibited.

Ptolemy was preeminently responsible for the geocentric cosmology that prevailed in the Islamic world and in medieval Europe. This was not due to the
Almagest
so much as a later treatise,
Hypotheseis tōn planōmenōn (Planetary Hypotheses
). In this work he proposed what is now called the Ptolemaic system, a unified system in which each heavenly body is attached to its own sphere and the set of spheres nested so that it extends without gaps from the Earth to the celestial sphere. The numerical tables in the
Almagest
(which enabled planetary positions and other celestial phenomena to be calculated for arbitrary dates)
had a profound influence on medieval astronomy, in part through a separate, revised version of the tables that Ptolemy published as
Procheiroi kanones (Handy Tables
). Ptolemy taught later astronomers how to use dated, quantitative observations to revise cosmological models.

Ptolemy also attempted to place astrology on a sound basis in
Apotelesmatika (Astrological Influences
), later known as the
Tetrabiblos
for its four volumes. He believed that astrology is a legitimate, though inexact, science that describes the physical effects of the heavens on terrestrial life. Ptolemy accepted the basic validity of the traditional astrological doctrines, but he revised the details to reconcile the practice with an Aristotelian conception of nature, matter, and change. Of Ptolemy's writings, the
Tetrabiblos
is the most foreign to modern readers, who do not accept astral prognostication and a cosmology driven by the interplay of basic qualities such as hot, cold, wet, and dry.

(b. 129 CE, Pergamum, Mysia, Anatolia [now Bergama, Tur.]—d.
c
. 216)

G
alen of Pergamum (Latin: Galenus) was a Greek physician, writer, and philosopher who exercised a dominant influence on medical theory and practice in Europe from the Middle Ages until the mid-17th century. His authority in the Byzantine world and the Muslim Middle East was similarly long-lived.