As for retroduction, it is itself an experiment. A retroductive research is an experimental research; and when we look upon Induction and Deduction from the point of view of Experiment and Observation, we are merely tracing in those types of reasoning their affinity to Retroduction. [—] To return to Retroduction, then, it begins with Colligation. Something corresponding to Iteration may or may not take place. And then comes an Observation. Not, however, an External observation of the objects as in Induction, nor yet an observation made upon the parts of a Diagram, as in Deduction; but for all that just as truly an observation. [—] The act of observation is the deliberate yielding of ourselves to that force majeure, - an early surrender at discretion, due to our foreseeing that we must, whatever we do be borne down by that power, at last. Now the surrender which we make in Retroduction, is a surrender to the Insistence of an Idea. The hypothesis, as the Frenchman says, c’est plus fort que moi. It is irresistible; it is imperative. We must throw open our gates and admit it at any rate for the time being.