Water flows downhill. A person can't be in two places at once. All animals live, die and stay dead. Such common-sense statements about the world are so basic that no encyclopedia bothers to include them.
But it is exactly what an encyclopedia omits that Douglas Lenat, a research scientist at Microelectronics and Computer Technology Corporation, in Austin, Tex., wants in his knowledge base, called ''Cyc.'' Cyc is the name for an M.C.C. project at known as ''large common sense knowledge-base construction,'' which Mr. Lenat is designing to augment other computer programs by giving them common sense.
A data base is simply an assemblage of facts stored in a central computer; a knowledge base is broader, containing information about logic, relationships and descriptions of behavior.
''We're trying to instill in Cyc what the writers of the Britannica assumed everybody already knows,''Mr. Lenat said.
The Cyc project began in 1984, the same year M.C.C. was formed as high-technology research and development cooperative. Mr. Lenat projected then that it would take 10 years to create a knowledge base of 100 million common-sense assertions.
When the Cyc project is completed its knowledge base will contain 4 billion or 5 billion bytes of information - 10,000 times larger than a typical expert system today. Expert systems - programs that attempt to mimic the decision-making and thought processes of expert human beings - are the most common type of artificial intelligence.
The work Mr. Lenat is doing does not come cheaply. Each of the seven shareholders in the Cyc project - Apple Computer Inc., Bell Communications Research Inc. (Bellcore), Control Data Corporation, Digital Equipment Corporation, Harris Corporation, Eastman Kodak Company and NCR Corporation - contribute $500,000 a year. With their investment, the participating companies gain the right to use whatever Mr. Lenat's research efforts achieve to develop and enhance their own computer products.
The problem with expert systems is that the programs operate in only one, specialized subject area, such as oil exploration. Mr. Lenat's research, if it succeeds, could give these programs greater breadth and flexibility. For instance, a computer program with a general understanding of the way the world works - and specific knowledge about a person's schedule, habits, past correspondence, job and so on - could answer that person's routine electronic mail on a variety of subjects.
Cyc's real distinction is not the amount of knowledge it includes, but the kind. Only about one-third of its memory will be devoted to standard encyclopedia-style information. The remaining two-thirds will contain rules of thumb (''People generally can't see through walls''); laws of physical objects (''A car parked at the airport will tend to stay parked there until you return from your destination''), and constraints on life (''Nothing can be older than its creator'').
In the constraints category, Mr. Lenat cites a self-evident proposition: No child can be older than its parents.
''People would never make the mistake that the child could be older than his parents,'' Mr. Lenat said. ''But the typical computer program would.''
Mr. Lenat is attempting to create a program that can be customized for particular environments or people. Cyc would be able to intelligently monitor other programs as well as the actions of the operator. For example, if a computer programmed to follow financial investments advised an individual to purchase a 20-year bond, Cyc might intercede and remind the computer user that in 10 years he may need the money to pay for his daughter's college tuition.
Mr. Lenat's sponsors are beginning to determine how to use his research. ''One idea,'' said Ed Freeman of U. S. West, a Bell operating company which participates in the Cyc project through Bellcore, ''is to develop a customer adviser assistant. It would give U. S. West personnel advice on solving customer problems on-line while they are working with customers.''
Digital is already testing Cyc to design computer systems for customers. John McDermott, head of the Digital's Artificial Intelligence Research Group, foresees advanced versions of Cyc that can assess the computing needs of Digital's customers and select components, like terminals, central processing units and memory, that would fulfill those needs.
To Mr. McDermott, Cyc's essential quality is that it brings a tremendous amount of knowledge and intelligence to a problem. If the goals of an organization are changing, a person could describe the current corporate structure to Cyc, which would retrieve from its knowledge examples of similarly organized companies that have succeeded or failed trying to attain similar goals. Cyc could replace the haphazard, time-consuming hunt through journals and periodicals.
Joseph Scullion, director of strategic planning at NCR, cannot yet point to any product that his company might derive from Cyc. ''But,'' he said, ''just being able to capture common-sense intelligence in a work station means that whatever application you run on it can be more complex. And lightly trained people can be more productive.''