QuestionsWhy is there something rather than nothing?There is no necessary being: the world can just be there
Why is there something rather than nothing?A brute fact

There is no necessary being: the world can just be there

Hume's critique of the necessary being

David Hume 1711 · 1776Neither causation nor induction can be proved: custom guides us.
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What it claims

Nothing obliges us to seek a reason for the world's existence outside it: the sequence of things can simply be there, with no ultimate reason.

Why

Whatever we conceive as existing we can conceive as not existing: no existence can be demonstrated, and the world may simply be there.

The same idea through history
David Hume1779Cleanthes answers Demea, who defends the a priori argument: existence cannot be demonstrated, and the whole needs no cause beyond its parts.
Bertrand Russell1948Russell takes up the idea on BBC radio: the universe is just there, and that is all.
Carl Sagan1980"If we say that God always existed, why not save a step, and conclude that the Universe always existed?" In Cosmos, Sagan takes up Hume's idea: if we accept an existence without beginning, nothing obliges us to place it in God rather than in the world.
In short

"There is no necessary being: the world can just be there" is an argument by David Hume (Dialogues concerning Natural Religion, IX, 1779, 1779) on the question "Why is there something rather than nothing?" (answer: a brute fact). Its claim: Nothing obliges us to seek a reason for the world's existence outside it: the sequence of things can simply be there, with no ultimate reason. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P6). AskPhilosophy presents 2 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Dialogues concerning Natural Religion, IX, 1779
Valid with an implicit premise

In the Dialogues, Demea defends the a priori proof: the world must have a reason, so a necessary being exists. Cleanthes answers by attacking the very idea of a necessary being. Whatever we conceive as existing, we can conceive as not existing: no existence is therefore self-contradictory to deny, nor demonstrable. And if something had to be necessary, why not the universe itself? Hume does not say the world has a hidden reason: he says we have no motive to demand one outside it. (In the Dialogues, Hume's own view is mostly read in Philo, but this argument is Cleanthes's.)

P1Nothing is demonstrable unless its contrary implies a contradiction.
P2Nothing that is distinctly conceivable implies a contradiction.
P3Whatever we conceive as existent, we can also conceive as non-existent.
P4If a being could exist necessarily, nothing prevents it from being the material universe: we do not know all the properties of matter.
P5In a chain of causes, each part is explained by the one before; uniting the parts into a whole is an arbitrary act of the mind, and explaining the parts is enough to explain the whole.
P6 · implicit premiseIf no being is demonstrable as necessary, the universe can take its place and the whole is explained by its parts, nothing obliges us to seek a reason outside the sequence of things.
Therefore nothing obliges us to seek a reason for the world's existence outside it: the sequence of things can simply be there, with no ultimate reason.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P6, the computer finds a world where no being is demonstrably necessary, where the universe could be, and where each part is explained, yet where the whole still has to be explained by something else: the conclusion does not follow.
Valid with P6. Hume does not state it this way, but he needs it: from "no necessary being is demonstrable" it does not yet follow that we need not look for a reason. The universe must also be able to stand in for a necessary being (P4), and the whole must require nothing more than its parts (P5). Supporters of Leibniz's argument mainly dispute P5; others dispute P1 (is necessity only a matter of logical contradiction?).
Analogy. Twenty grains of sand: once the origin of each is given, asking "and where does the heap of twenty come from?" adds no further question. This is Hume's example, with twenty particles of matter.

Objections

2

Each objection is a conversation: click to see who replies and where the debate stands.

D
Demea 1779 · attacks premise 5

Each effect is explained by the cause before it, but the whole eternal chain, taken together, is determined by nothing. It remains reasonable to ask why this succession has existed from eternity, and why something rather than nothing. (Hume points in a note to Clarke: this is the argument from contingency of Clarke and Leibniz.)

Hume replies
Hume (Cleanthes) 1779 · Cleanthes's reply
In an eternal succession, it seems absurd to ask for a general cause or first author: the causal relation implies priority in time and a beginning of existence, which what exists from eternity does not have.
Alexander Pruss 2006
Summary: Pruss defends the principle of sufficient reason and holds that the demand for explanation applies to the whole of contingent facts as well, not merely to each. Seeking the reason of the whole is therefore not a mere game of the mind, and the absence of a beginning in time does not remove it.
Where the debate stands (our summary)Core disagreement: is the principle of sufficient reason true without exception, including for the whole of all things? For Hume the demand for explanation stops at the parts; for Clarke, Leibniz and Pruss it applies to the whole. Open point: no widely accepted answer.
Leibniz 1714 · attacks premise 4

The material world cannot be the necessary being: matter is in itself indifferent to motion and to rest, and to one particular motion rather than another. We therefore cannot find in it the reason of motion, still less of one particular motion.

no reply
see the argument
No reply possible: objection raised in 1714, after the author's death (1776).
The debate goes on (our summary) Hume writes after Leibniz and does not cite him here. But Cleanthes examines the classic argument from the contingency of matter (matter can be conceived annihilated or otherwise): he answers that the same argument tells against God, and that unknown qualities might make matter necessary (Dialogues, IX). Leibniz holds that the reason for an indifferent matter is not found in itself.
Where the debate stands (our summary)Core disagreement: whether matter does or does not contain a reason for its existence and motion. Hume judges we do not know matter well enough to rule it out; Leibniz judges that its nature shows it to be indifferent.