348 NOVUM ORGANUM. 



heard tell of a crow, which, being nearly dead with thirst during a 

 great drought, saw some water in the hollow of a tree, and finding the 

 opening too narrow for it to enter, threw in a number of pebbles, until 

 the water rose high enough for it to drink, which afterwards passed 

 into a proverb. 



In like manner let the Nature inquired into be Visibility. It seems 

 to be a perfectly true and safe distinction which is made between 

 Light, as visible originally, and affording the primary means of seeing, 

 and Colour, as being visible secondarily, and not to be discerned 

 without light ; so that it appears to be nothing but an image or modi 

 fication of Light. And yet, on either side in this case, there appear 

 to be Instances of Alliance ; as snow in large quantities, and the flame 

 of sulphur ; in one of which we see Colour primarily giving Light, in 

 the other, Light verging towards Colour. 



xxxvi. Among Prerogative Instances we shall put in the fourteenth 

 place, Instances of t lie Cross, the word being borrowed from the Crosses, 

 which are set up where roads meet, to indicate and mark the different 

 directions. These we call also Decisive and Judicial, and, in some 

 cases, Oracular and Commanding Instances. Their method is as fol 

 lows. When, in the investigation of any Nature, the Understanding 

 is placed, so to speak, in equilibria, so that it is uncertain to which of 

 two, or sometimes more Natures, the cause of the Nature investigated 

 ought to be attributed or assigned, on account of the frequent and 

 ordinary concurrence of several Natures ; Instances of the Cross show 

 the union of one of the Natures with the Nature investigated to be 

 sure and indissoluble, that of the other to be changeable and 

 separable ; thus the question is decided, and the former Nature is 

 received as the cause, while the latter is dismissed and rejected. And 

 so Instances of this kind supply very great light, and are of great 

 authority ; the course of Interpretation sometimes ending in them, 

 and being accomplished by them. Sometimes these Instances of the 

 Cross are discovered by chance among those already noticed ; but 

 they are for the most part new, and industriously and designedly 

 sought out and applied, and discovered only by unremitting and 

 active diligence. 



For example, let the Nature inquired into be the Flow and Ebb of 

 the Sea, which is repeated twice in the day, and occupies six hours in 

 each advance and retreat, with a certain difference corresponding 

 with the motion of the moon ; the following is an example of two 

 ways meeting with respect to this Nature. 



This motion must necessarily be caused either by the advance and 

 retreat of the waters, as water shaken in a basin wets one side and 

 leaves the other bare ; or by the rising of the waters from the deep, 

 and their subsidence, after the manner of water which boils and again 

 subsides. And the question arises, to which of these three causes 

 should the flow and ebb be assigned? Now, if the first assertion be 

 admitted, it must happen that when there is flood-tide in the sea on 

 the one side, there is at the same time an ebb somewhere on the 

 Other ; to this issue, therefore, the inquiry is brought. But it has 



