are demonstrated by the first which are their causes, the first are in
their turn demonstrated by the last which are their effects. Nor must it
be imagined that I here commit the fallacy which the logicians call a
circle; for since experience renders the majority of these effects most
certain, the causes from which I deduce them do not serve so much to
establish their reality as to explain their existence; but on the
contrary, the reality of the causes is established by the reality of the
effects. Nor have I called them hypotheses with any other end in view
except that it may be known that I think I am able to deduce them from
those first truths which I have already expounded; and yet that I have
expressly determined not to do so, to prevent a certain class of minds
from thence taking occasion to build some extravagant philosophy upon what
they may take to be my principles, and my being blamed for it. I refer to
those who imagine that they can master in a day all that another has taken
twenty years to think out, as soon as he has spoken two or three words to
them on the subject; or who are the more liable to error and the less
capable of perceiving truth in very proportion as they are more subtle and
lively. As to the opinions which are truly and wholly mine, I offer no
apology for them as new, -- persuaded as I am that if their reasons be
well considered they will be found to be so simple and so conformed, to
common sense as to appear less extraordinary and less paradoxical than any
others which can be held on the same subjects; nor do I even boast of being
the earliest discoverer of any of them, but only of having adopted them,
neither because they had nor because they had not been held by others,
but solely because reason has convinced me of their truth.

Though artisans may not be able at once to execute the invention which is
explained in the "Dioptrics," I do not think that any one on that account
is entitled to condemn it; for since address and practice are required in
order so to make and adjust the machines described by me as not to