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a scientific and philosophic rule that entities should not be multiplied unnecessarily which is interpreted as requiring that the simplest of competing theories be preferred to the more complex or that explanations of unknown phenomena be sought first in terms of known quantities.
why this sentence uses be, not "is"? many thanks.......
The sentence is written in the subjunctive mood. If you are not familiar with this, start with
Subjunctive mood - Wikipedia, the free encyclopedia