Absolutely nothing at all
The fool saith in his heart that there is no empty set. But if they were so, then the set of all such sets would be empty and hence it would be the empty set.
Wesley Salmon

The ultimate goal of mathematics is to eliminate all need for intelligent thought.
Ronald Graham, Donald Knuth & Owen Patashnik

A set is a set
(you bet; you bet!)
And nothing could not be a set,
You bet!
That is, my pet
Until you’ve met
My very special set.
Bruce Reznick

Richard Cleveland
We can’t be assured if a full set
Or even a reasonably dull set.
It wouldn’t be clear
That there’s any set here,
Unless we assume there’s a null set.

Bertrand Russell
Pure mathematics consists entirely of such asseverations as that, if such and such a proposition is true of anything, then such and such another proposition is true of that thing. It is essential not to discuss whether the first proposition is really true, and not to mention what the anything is of which it is supposed to be true…If our hypothesis is about anything and not about some one or more particular things, then our deductions constitute mathematics. Thus mathematics may be defined as the subject in which we never know what we are talking about, nor whether what we are saying is true.

You cannot have first space and then things to put into it, any more than you can first have a grin and then a Cheshire cat to fit into it.
Alfred North Whitehead

If an elderly, but distinguished scientist says that something is possible he is almost certainly right, but if he says that is impossible he is probably wrong.
Arthur C. Clarke

I love cosmology: there’s something uplifting about viewing the entire universe as a single object with a certain shape. What entity, short of God, could be nobler or worthier of a man’s attention than the cosmos itself? Forget about interest rates, forget about war and murder, let’s talk about space.
Rudy Rucker

There is an element of tragedy in the life-story of the ether. First we had luminiferous ether. It’s freely given services as midwife and nurse to the wave theory of light and to the concept of field were of incalculable value to science. But after its charges had grown to man’s estate it was ruthlessly, even joyfully, cast aside, its faith betrayed and its last days embittered by ridicule and ignominy. Now that it is gone it still remains unsung.
Banesh Hoffman

This [quantum] theory reminds me a little of the system of delusions of an exceedingly intelligent paranoic, concocted of incoherent elements of thoughts.
Albert Einstein

Niels Bohr
Anyone who is not shocked by quantum theory has not understood it.

Einstein
The quantum theory gives me a feeling very much like yours. One really ought to be ashamed of its success, because it has been obtained in accordance with the Jesuit maxim: ‘Let not thy left hand know what thy right hand doeth’

Hendrick Kramers
The theory of quanta is similar to other victories in science; for some months you smile at it, and then for years you weep.

God is in the atoms…A superposition, if you like. Or whether you don’t like actually, that’s what it’s called. A superposition is like God in that the quantum object occupying a number of different states simultaneously can be everywhere at once. A superposition is a kind of immanence. Without these superpositions, quantum objects would simply crash into each other and solid matter could not possibly exist.
Philip Kerr

The universe is merely a fleeting idea in God’s mind – a pretty uncomfortable thought, particularly if you’ve just made a down payment on a house.
Woody Allen

I never predict anything and I never will do.
Paul Gazza Gascoigne
(you’ve gotta love his humour)

It does not do to leave a dragon out of your calculations, if you live near him.
J.R.R. Tolkien
(most certainly)

Georges Lemaitre
Since I have introduced this term I have always had a bad conscience. But at that time I could see no other possibility to deal with the fact of the existence of a finite mean density of matter. I found it very ugly indeed that the field force law of gravitation should be composed of two logically independent terms which are connected by addition. About the justification of such feelings concerning logical simplicity it is difficult to argue. I cannot help to feel it strongly that I am unable to believe that such an ugly thing should be realized in Nature.

But we shall all be changed, in a moment, in the twinkling of an eye…
St Paul

Cats, no less liquid than their shadows,
Offer no angles to the wind.
They slip, diminished, neat, through loopholes
Less than themselves.
A.S.J. Tessimond

Every public action, which is not customary, either is wrong, or, if it is right, is a dangerous precedent. It follows that nothing should ever be done for the first time.
Francis Cornford

Leibniz
I want to know why the universe exists…Whence comes it, and how can it be understood. That anything exists?...Now, if I push these questions aside and go straight to what hides behind them, this is what I find:- Existence appears to me like a conquest over nought…If I ask myself why bodies or minds exist rather than nothing, I find no answer; but that a logical principle, such as A=A, should have the power of creating itself, triumphing over the nought throughout eternity, seems to be natural…Suppose, then, that the principle on which all things rest, and which all things manifest, possesses an existence of the same nature as that of the definition of the circle, or as that of the axiom A=A: the mystery of existence vanishes…

Nicholas Rescher
Getting real existence from pure logic is just too much of a conjuring trick. That sort of hat cannot contain rabbits.

We are the music-makers
And we are the dreamer of dreams,
wandering by lone sea-breakers,
And sitting by desolate streams;
World-losers and world-forsakers,
On whom the pale moon gleams:
Yet we are the movers and shakers
Of the world forever, it seems.
Arthur O’Shaughnessy, ‘Ode”

The star nor sun shall waken,
Nor any change of light:
Nor sound of waters shaken,
Nor any sound or sight:
Nor wintry leaves nor vernal,
Nor days not things diurnal,
Only the sheep eternal
In an eternal night.
Algernon Swinburne

(Reminds me very much of: If you take good care of me then I’ll be your caretaker. You be the maker of the peace – I’ll be the peacemaker.) – Purely coincidental? You decide.