XXIII

Among Prerogative Instances I will next place Migratory Instances. They are those in which the nature in question is in the process of being produced when it did not previously exist, or on the other hand of disappearing when it existed before. And therefore, in either transition, such instances are always twofold, or rather it is one instance in motion or passage, continued till it reaches the opposite state. Such instances not only accelerate and strengthen the exclusive process, but also drive the affirmative or form itself into a narrow compass. For the form of a thing must necessarily be something which in the course of this migration is communicated, or on the other hand which in the course of this migration is removed and destroyed. And though every exclusion promotes the affirmative, yet this is done more decidedly when it occurs in the same than in different subjects. And the betrayal of the form in a single instance leads the way (as is evident from all that has been said) to the discovery of it in all. And the simpler the migration, the more must the instance be valued. Besides, migratory instances are of great use with a view to operation, because in exhibiting the form in connection with that which causes it to be or not to be, they supply a clear direction for practice in some cases; whence the passage is easy to the cases that lie next. There is, however, in these instances a danger which requires caution; viz., lest they lead us to connect the form too much with the efficient, and so possess the understanding, or at least touch it, with a false opinion concerning the form, drawn from a view of the efficient/But the efficient is always understood to be merely the vehicle that carries the form. This is a danger, however, easily remedied by the process of exclusion legitimately conducted.

I must now give an example of a migratory instance. Let the nature to be investigated be whiteness. An instance migrating to production or existence is glass whole and pounded. Again, simple water and water agitated into froth. For glass and water in their simple state are transparent, not white, whereas pounded glass and water in froth are white, not transparent. We must therefore inquire what has happened to the glass or water from this migration. For it is obvious that the form of whiteness is communicated and conveyed by that pounding of the glass and that agitation of the water. We find, however, that nothing has been added except the breaking up of the glass and water into small parts, and the introduction of air. But we have made no slight advance to the discovery of the form of whiteness when we know that two bodies, both transparent but in a greater or less degree (viz., air and water, or air and glass), do when mingled in small portions together exhibit whiteness, through the unequal refraction of the rays of light.

But an example must at the same time be given of the danger and caution to which I alluded. For at this point it might readily suggest itself to an understanding led astray by efficient causes of this kind, that air is always required for the form of whiteness, or that whiteness is generated by transparent bodies only — notions entirely false, and refuted by numerous exclusions. Whereas it will be found that (setting air and the like aside) bodies entirely even in the particles which affect vision are transparent, bodies simply uneven are white; bodies uneven and in a compound yet regular texture are all colors except black; while bodies uneven and in a compound, irregular, and confused texture are black. Here then I have given an example of an instance migrating to production or existence in the proposed nature of whiteness. An instance migrating to destruction in the same nature of whiteness is froth or snow in dissolution. For the water puts off whiteness and puts on transparency on returning to its integral state without air.

Nor must I by any means omit to mention that under migratory instances are to be included not only those which are passing toward production and destruction, but also those which are passing toward increase and decrease; since these also help to discover the form, as is clear from the above definition of form and the Table of Degrees. The paper, which is white when dry, but when wetted (that is, when air is excluded and water introduced) is less white and approaches nearer to the transparent, is analogous to the above given instances.