René Descartes 
Six metaphysical meditations 

Some years past I perceived how many Falsities I admitted as Truths in my Younger years, and how Dubious those things were which I raised from thence; and therefore I thought it requisite (if I had a designe to establish any thing that should prove firme and permanent in sciences) that once in my life I should clearly cast aside all my former opinions, and begin a new from some First principles. But this seemed a great Task, and I still expected that maturity of years, then which none could be more apt to receive Learning; upon which Account I waited so long, that at last I should deservedly be blamed had I spent that time in Deliberation which remain’d only for Action.

This day therefore I conveniently released my mind from all cares, I procured to my self a Time Quiet, and free from all Business, I retired my self Alone; and now at length will I freely and seriously apply my self to the General overthrow of all my former Opinions.

To the Accomplishment of Which, it will not be necessary for me to prove them all false (for that perhaps I shall never atcheive) But because my reason perswades me, that I must withdraw my assent no less from those opinions which seem not so very certain and undoubted, then I should from those that are Apparently false, it will be sufficient if I reject all those wherein I find any Occasion of doubt.