Roger Bacon (Doctor Mirabilis)

(c.1214 – c.1294)

"I believe that humanity shall accept as an axiom for its conduct the principle for which I have laid down my life: the right to investigate. It is the credo of free men this opportunity to try, this privilege to err, this courage to experiment anew. We scientists of the human spirit shall experiment, experiment, ever experiment."

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"And standing by himself because of his distinctive genius was Roger Bacon (circa 1210 to circa 1293), a Franciscan of Oxford, the father of modern experimental science. His name deserves a prominence in our history second only to that of Aristotle." [Wells, H. G. (1922). A Short History of the World]
From Opus Maius:
"I now wish to unfold the principles of experimental science, since without experience nothing can be sufficiently known. For there are two modes of acquiring knowledge, namely by reasoning and experience. Reasoning draws a conclusion and makes us grant the conclusion, but does not make the conclusion certain, nor does it remove doubt so that the mind may rest on the intuition of truth, unless the mind discovers it by the path of experience; since many have the arguments relating to what can be known, but because they lack experience they neglect the arguments, and neither avoid what is harmful nor follow what is good. For if a man who has never seen fire should prove by adequate reasoning that fire burns and injures things and destroys them, his mind would not be satisfied thereby, nor would he avoid fire, until he placed his hand or some combustible substance in the fire, so that he might prove by experience that which reasoning taught. But when he has had actual experience of combustion his mind is made certain and rests in full light of truth. Therefore reasoning does not suffice, but experience does."
"Quatuor vero sunt maxima comprehendendæ veritatis offendicula, quæ omnem quemcumque sapientem impediunt, et vix aliquem permittunt ad verum titulum sapientiæ pervenire: videlicet fragilis et indignæ auctoritatis exemplum, consuetudinis diuturnitas, vulgi sensus imperiti, et propriæ ignorantiæ occultatio cum ostentatione sapientiæ apparentis." [Roger Bacon. (1267). Opus Maius. (alt. Opus Majus)]

The Latin (above) has been variously translated as:

Four chief sources of ignorance: respect for authority; custom; the sense of the ignorant crowd; and the vain, proud unteachableness of our dispositions. (The Latter Rain by Jay Atkinson)

Four very significant stumbling-blocks in the way of grasping the truth, which hinder every man (sic) however learned, and scarcely allow anyone to win a clear title to wisdom, namely: the example of weak and unworthy authority, longstanding custom, the feeling of the ignorant crowd, and the hiding of our own ignorance while making a display of our apparent knowledge.

Four causes of error: the example of unreliable and unsuited authorities, the long duration of habit, the opinion of the ignorant masses, and the propensity of humans for disguising ignorance by the display of psuedo-wisdom. (Internet Encyclopedia of Philosophy)

Four chief obstacles in grasping truth: submission to faulty and unworthy authority, influence of custom, popular prejudice, and concealment of our own ignorance.

Four barriers blocking the road to truth: submission to unworthy authority, the influence of custom, popular prejudice, and concealment of one's ignorance with a technical show of wisdom. (The Encyclopedia of Philosophy)

Four obstacles to real wisdom and truth, viz. errors and their sources (the four general causes of human ignorance): the example of weak and unreliable authority; continuance of custom, regard to the opinion of the unlearned, and concealing one's own ignorance, together with the exhibition of apparent wisdom. (The Catholic Encyclopedia)

Four stumbling blocks to truth: unworthy authority, custom and convention, unrefined sensibilities, and the ostentation of seeming wisdom. (This is the way I learned them from Wanda's wall hanging via Ian Reid.)

Roger Bacon's Seven Degrees of Illumination