pre-Socratics Earliest Greek philosophers (those who preceded Socrates) whose attention to questions about the origin and nature of the physical world has led to their being called cosmologists or naturalists. Among the most significant were the Milesians Thales, Anaximander, and Anaximenes, Xenophanes of Colophon, Parmenides, Heracleitus of Ephesus, Empedocles, Anaxagoras, Democritus, Zeno of Elea, and Pythagoras.
Preakness Stakes One of the three classic U.S. horse races making up the Triple Crown. It is held annually in mid-May at Baltimore’s Pim¬ lico Race Course. The course distance is l 3 /i6 mi (1.9 km). The field is limited to 3-year-old Thoroughbreds.
Prebisch Vpra-bishV Raul (b. April 17, 1901, Tucuman, Arg.—d. April 29, 1986, Las Vertientes, near Santiago, Chile) Argentine economist and statesman. Serving in various positions in Argentine government and aca¬ demia, he advised developing countries to stimulate domestic manufac¬ turing to reduce their reliance on imports and thus their dependence on the industrialized nations. He also advocated the economic integration of Latin America to achieve economies of scale and land reform to reduce the income inequalities that impeded the growth of the domestic market. As the influential head of the UN’s Economic Commission for Latin America, he addressed inequities in the trade relationships between wealthy and poor nations.
Precambrian time Interval of geologic time from c. 3.8 billion years ago, the age of the oldest known rocks, to 544 million years ago, the beginning of the Cambrian Period. This interval represents more than 80% of the geologic record and thus provides important evidence of how the continents evolved. The Precambrian is divided into the Archean and Proterozoic eons, with the boundary between them at 2.5 billion years ago. It was originally defined as the era that predated the emergence of life in the Cambrian Period. It is now known, however, that life on Earth had begun by the early Archean. Soft-bodied organisms without skeletons began to appear toward the end of the Precambrian.
precast concrete Concrete cast into structural members under factory conditions and then brought to the building site. A 20th-century develop¬ ment, precasting increases the strength and finish durability of the mem¬ ber and decreases time and construction costs. Concrete cures slowly; the design strength is usually reached 28 days after initial setting. Using pre¬ cast concrete eliminates the lag between the time on-site concrete is placed and the time it can carry loads. Precast concrete components include slabs, beams, columns, walls, stairways, modular boxes, and even kitchens and bathrooms with precast fixtures. See also prefabrication.
precession Phenomenon associated with the action of a gyroscope or a spinning top and consisting of a comparatively slow rotation of the axis of rotation of a spinning body about a line intersecting the spin axis. It arises as a result of external torque acting on the body. One example of precession is the smooth, slow circling of a spinning top (the uneven wobbling is called nutation). Precession of the earth’s axis of rotation is the reason that the positions of celestial bodies appear to drift systemati¬ cally with the passage of time. See also precession of the equinoxes.
Tibetan prayer wheel, gilt silver, 18th— 19th century; in the Seattle (Washing¬ ton) Art Museum.
COURTESY OF THE SEATTLE ART MUSEUM, WASHINGTON, EUGENE FULLER MEMORIAL COLLECTION
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1542 I precipitation ► prelude
precipitation All liquid and solid water particles that fall from clouds and reach the ground, including drizzle, rain, snow, ice crystals, and hail. The essential difference between a precipitation particle and a cloud par¬ ticle is size; an average raindrop has a mass equivalent to that of about one million cloud droplets. Precipitation elements (ice crystals or drop¬ lets that form around soluble particles such as salt) form directly from the vapour state and get larger through collision and coalescence. Eventually they become large enough to respond to gravity, and they fall to the ground.
Precisionism Smooth, precise technique used primarily in the 1920s by several U.S. painters in representational canvases depicting sharply defined forms, such as urban skylines; the industrial landscape of facto¬ ries and smokestacks, buildings, and machinery; and country landscapes with grain elevators and barns or empty desert and sky. The scenes are always devoid of people or signs of human activity. Precisionism is a “cool” art, which keeps the viewer at a distance. It had its origins in Cub¬ ism, Futurism, and Orphism; in turn it influenced Pop art. Though it was not a school or movement with a formal program, the Precisionist artists, including Charles Demuth and Georgia O'Keeffe, often exhibited together.
predation Form of food getting in which one animal, the predator, eats an animal of another species, the prey, immediately after killing it or, in some cases, while it is still alive. Most predators are generalists; they eat a variety of prey species. Specialist predators, such as anteaters, eat only one or a few prey species. Cannibalism is a type of predation in which an animal eats another of its own species. Seed consumption is also con¬ sidered predation because the entire living embryo of a plant is destroyed.
predestination In Christianity, the doctrine that God has long ago determined who will be saved and who will be damned. Three types of predestination doctrine have developed. One doctrine holds that God singled out the saved because he foresaw their future merits. A second doctrine (often identified with John Calvin) states that from eternity God has determined the saved and the damned, regardless of their merit or lack thereof. A third doctrine, set forth by Thomas Aquinas and Martin Luther, ascribes salvation to the unmerited grace of God but links the lack of grace to sin. In Islam, issues of predestination and free will were argued extensively. The Mutazila held that God would be unjust if he predestined all human actions; the Ashariya advocated a strict predestination that became the mainstream Islamic view.
predicate calculus Part of modern symbolic logic which systemati¬ cally exhibits the logical relations between propositions involving quan¬ tifiers such as “all” and “some.” The predicate calculus usually builds on some form of the propositional calculus and introduces quantifiers, indi¬ vidual variables, and predicate letters. A sentence of the form “All F’s are either G’s or H’s” is symbolically rendered as (Vx)[Fx z> (Gx v Hx)J, and “Some F’s are both G’s and H’s” is symbolically rendered as (3x)[Fx a (Gx a Hx)]. Once conditions of truth and falsity for the basic types of propositions have been determined, the propositions formulable within the calculus are grouped into three mutually exclusive classes: (1) those that are true on every possible specification of the meaning of their predicate signs, such as “Everything is F or is not F”; (2) those false on every such specification, such as “Something is F and not F”; and (3) those true on some specifications and false on others, such as “Something is F and is G.” These are called, respectively, the valid, inconsistent, and contingent propositions. Certain valid proposition types may be selected as axioms or as the basis for rules of inference. There exist multiple complete axi- omatizations of first-order (or lower) predicate calculus (“first-order” meaning that quantifiers bind individual variables but not variables rang¬ ing over predicates of individuals). See also logic.
preeclampsia V.pre-i-'klamp-se-oV and eclampsia Hypertensive conditions unique to pregnancy. Preeclampsia is marked by hypertension, protein in the urine, and hand and face edema, which develop late in preg¬ nancy or soon after. Persistent hypertension compromises the fetus’s blood supply and damages the mother’s kidneys. Monitoring of blood pressure and weight gain may detect it before symptoms (headaches, visual disturbances, stomach pain) begin. Eclampsia follows in about 5% of cases, with convulsions that pose a serious threat to mother and child. It can usually be prevented by special diets, drugs, and limited activity or early delivery.