RATIONAL LEGAL DECISIONMAKING REQUIRES THAT ONE ASSESS WHAT IS KNOWN, WHAT INFORMATION FURTHER INQUIRY MIGHT DISCOVER, THE COST OF SUCH INQUIRY, THE TIME REQUIRED, THE PROBABILITY THAT SUCH INFORMATION MIGHT WEAKEN TENTATIVE CONCLUSIONS, AND THE MEASURE OF SUCH POTENTIAL WEAKENING. THERE ARE THREE FUNDAMENTAL CONCEPTS AT THE CORE OF OPEN-ENDED FACTURAL INQUIRY. FIRST, TRUE PROBABILITY IS A CERTAINTY. THIS ASSUMES A DEFINITION OF PROBABILITY IN WHICH THERE IS A GIVEN NUMBER (N) OF EQUALLY POSSIBLE KNOWN CAUSES; GIVEN THAT CERTAIN OF THESE CAUSES (P) BELONG TO A PARTICULAR CLASS (A), THEN THE PROBABILITY THAT THE CAUSE OF THE EVENT WAS A MEMBER OF CLASS A IS P/N. SECOND, WHAT IS KNOWN DEPENDS ON WHAT IS NOT KNOWN; THAT IS, THE STRENGTH OF A GIVEN PROBABILITY FRACTION OF THE FORM P/N WILL DEPEND ON THE QUANTITY AND QUALITY OF THE INFORMATION NOT CONSIDERED. THUS, CONFIDENCE IN A BELIEF DEPENDS IN PART ON AN EVALUATION OF PARTIAL BELIEFS ACCORDING TO THE CONFIDENCE THE ACTOR HAS IN THOSE BELIEFS AND ON THE ASSUMPTION THAT THERE IS A LIMIT TO THE AMOUNT OF UNKNOWN INFORMATION. THESE THREE CONCEPTS ARE DISCUSSED AND EXEMPLIFIED AS THEY APPLY TO THE DECISIONMAKING PROCESS. USING PROBABILITY IN THE TOSSING OF A FAIR COIN, A NUMBER OF ALGEBRAIC FORMULAS ARE DERIVED FOR DECISIONMAKING IN TERMS OF MINIMUM AND MAXIMUM EXPECTED COSTS OF A CERTAIN DECISION AND ITS OPPOSITE BEFORE AND AFTER INQUIRY FOR ADDITIONAL INFORMATION. ON THE BASIS OF THESE FORMULATIONS, THE MAXIMUM VALUE OF THE OUTSIDE BOUND OF INQUIRY (X) IS DIRECTLY PROPORTIONAL TO THE COST OF ERROR AND INVERSELY PROPORTIONAL TO THE SUM (K) OF THE INCREMENTAL COSTS OF THE INQUIRY AND OF THE DELAY AT EACH STEP OF THE INQUIRY. BY ESTIMATING THESE TWO AMOUNTS (X AND K), IT IS POSSIBLE TO ESTABLISH A GUIDE TO DETERMINING WHEN FURTHER INQUIRY IS NO LONGER JUSTIFIED BECAUSE OF THE DIMINISHED RETURN ON THE INQUIRY EFFORTS. THE NEED FOR PRECISE RULES TO MEASURE THE ADEQUACY OF INFORMATION ON WHICH ADMINISTRATIVE COST-BENEFIT DECISIONS ARE TO BE MADE, AS OPPOSED TO RELIANCE ON HUNCHES AND THE LAWYER'S AND CLIENT'S RESOURCES, IS REITERATED. NOTES ARE INCLUDED. (JAP)