Christiaan Huygens quotes:

These Gentlemen must be told, that they take too much upon themselves when they pretend to appoint how far and no farther Men shall go in their Searches, and to set bounds to other Mens Industry; as if they knew the Marks that God has placed to Knowledge...

It's evident God had no design to make a particular Enumeration in the Holy Scriptures, of all the Works of his Creation.

Now since in so many Things they... agree, what can be more probable than that in others they agree too; and that the other Planets are as beautiful and as well stock'd with Inhabitants as the Earth? Or what shadow of Reason can there be why they should not?

Here we may mount from this dull Earth, and viewing it from on high, consider whether Nature has laid out all her Cost and Finery upon this small Speck of Dirt.

The world is my country. Science my religion.

What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths....

We shall be less apt to admire what this World calls Great, shall nobly despise those Trifles the generality of Men set their Affections on, when we know that there are a multitude of such Earths inhabited and adorned as Well as our own.

It is surrounded by a thin flat ring, inclined to the ecliptic, and nowhere touches the body of the planet.

There are many degrees of Probable, some nearer Truth than others, in the determining of which lies the chief exercise of our Judgment.

I do not mind at all that [Newton] is not a Cartesian provided he does not offer us suppositions like that of attraction.

I believe that we do not know anything for certain, but everything probably.

How vast those Orbs must be, and how inconsiderable this Earth, the Theatre upon which all our mighty Designs, all our Navigations, and all our Wars are transacted, is when compared to them. A very fit consideration, and matter of Reflection, for those Kings and Princes who sacrifice the Lives of so many People, only to flatter their Ambition in being Masters of some pitiful corner of this small Spot.

One may conceive light to spread successively, by spherical waves.

The rest of the planets have their dress and furniture, nay and their inhabitants too, as well as this Earth of ours.

I esteem his understanding and subtlety highly, but I consider that they have been put to ill use in the greater part of his work, where the author studies things of little use or when he builds on the improbable principle of attraction. Writing about Newton's Principia. Huygens had some time earlier indicated he did not believe the theory of universal gravitation, saying it 'appears to me absurd.'

I had not thought of this regular decrease of gravity, namely that it is as the inverse square of the distance; this is a new and highly remarkable property of gravity.

...great difficulties are felt at first and these cannot be overcome except by starting from experiments .. and then be conceiving certain hypotheses ... But even so, very much hard work remains to be done and one needs not only great perspicacity but often a degree of good fortune.