THE HEARINGS conducted last week by Sen. John P. East's Judiciary subcommittee purported to seek an answer to that old question: When does human life begin? But Sen. East and a few colleagues knew the answer they wanted before the hearings got under way. They were out to gather sufficient scientific evidence that human life begins at the moment of conception to justify a congressional finding of that as fact.

They intend to interpret the meaning of the word "person" as it is used in the 14th Amendment in this light. By declaring that a "person" exists once a human egg is fertilized, their proposed legislation would permit the states to declare abortion -- with no exceptions -- to be murder and to outlaw some forms of birth control as instruments of murder.

In a society as sharply divided as this one is on abortion and birth control, the explosiveness of such legislation is obvious. The ramifications, however, run far beyond those two subjects.

If, for example, Congress has the power to define the word "person" in the 14th Amendment, it would logically follow that Congress can also define the other key words -- life, liberty, property, due process of law -- that occur in the same sentence. If Congress can do that by majority vote (with the concurrence of the president) as the East bill asserts, it has found a way to change the meaning of the Constitution without amending it.

Perhaps the simplest example involves that word "person." The Supreme Court ruled a century ago that corporations are persons for 14th Amendment purposes. If Congress has the power to say a fertilized egg is a person, does it have power to say a corporation is not a person? If it does, many of the legal rights around which the business world turns rest only on the reluctance of Congress to pass such a law.

One argument is that the East proposal merely defines a word that the court has refused to decline specifically. But the justices have never specifically defined liberty or due process or equal protection, either. These are terms that have been regarded as evolving concepts, subject to change as knowledge and public sentiment change. If Congress can substitute its definition for that of the court on one word, it would seem able to substitute its definition of the others. A simple majority in Congress, then, would replace the court as the final arbiter of the Constitution, subject to no reversal save by constitutional amendment.

The East proposal would bring so profound a change to the existing system of government that every member should spend days, if not weeks, studying it -- if it is to be considered at all -- not in terms of when life begins but in terms of where the power of Congress should end in interpreting the Constitution.