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INTRO D U C T I O N.

zi. The first is, that more causes are not to be receivedinto philosophy, than are sufficient to explain the appearancesof nature, ^hat this rule is approved of unanimously, is e-vident from those expressions so frequent among all philoso-phers, that nature does nothing in vain; and that a varie-ty of means, where fewer would suffice, is needless. Andcertainly there is the highest reason for complying with thisrule. For should we indulge the liberty of multiplying,without necessity, the causes of things, it would reduceall philosophy to mere uncertainty ; since the only proof,which we can have, os the existence of a cause, is the ne-cessity of it for producing known effects. Therefore whereone cause is sufficient, is there really should in nature betwo, which is in the last degree improbable, we can have nopossible means of knowing it, and consequently ought not totake the liberty os imagining, that there are more than one.

z 3. The second precept is the direct consequence os thefirst, that to like effects are to be ascribed the same causes.For instance, that respiration in men and in brutes is broughtabout by the same means; that bodies descend to the earthhere in E u r o p e, and in A m e r i c a from the same principle;that the light os a culinary fire, and os the sun have the samemanner os production; that the reflection os light is effected inthe earth, and in the planets by the same power; and the like.

24. The third of these precepts has equally evident rea-son for it. It is only, that those qualities, which in the samebody can neither be lessened nor increased, and which belong

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