Wikipedia:But there must be sources!

You may be confident that sources exist, but asserting this without proof is unlikely to convince anyone who believes that they don't. They may well have reached that conclusion by searching for references and failing to find any. Closing administrators on AfD debates will frequently afford unsupported assertions less weight. The best and most reliable way of convincing both doubters and the closing administrator is to actually provide the requested sources rather than simply declaring you're sure they must be out there somewhere.

Wikipedia's verifiability policy is one of its core content policies and demands that all material included in the encyclopedia must be sourced, or it may be challenged and removed. While some editors believe that, strictly speaking, this right of challenge only extends to material thought to be factually incorrect, in practice material is challenged on a variety of other grounds including notability concerns, relevance, undue weight, original research, etc. Articles can be, and frequently are, removed on these grounds. The burden of proof is on those who add or defend the contentious material to provide sources that satisfy the concerns of the challenging editor.

Insisting the sources must exist without being able to provide them is generally to be avoided in deletion discussions. Hypothetical examples include:
 * Keep – This is obviously notable, so it could be referenced. Prejudger 01:01, 1 January 2001 (UTC)
 * Keep – There must be plenty of sources. Presumer 01:01, 1 January 2001 (UTC)
 * Keep – You should find sources, instead of deleting it. ItsUpToYou 01:01, 1 January 2001 (UTC)
 * Keep – There are 20,000 google hits for some or all of the words in the title so surely there are sources. NeedleHaystack 01:01, 1 January 2001
 * Keep – You need to show there are no sources. ProveANegative 01:01, 1 January 2001
 * Keep – I expect that sourcing is available to verify basic facts Expector 01:01, 1 January 2001
 * Keep – Other people will be able to provide references. HaveFaith 01:01, 1 January 2001
 * Keep – I've seen sources but I won't tell you what they are or where I found them. YaGottaBelieveMe 01:01, 1 January 2001

We keep articles because we know they have sources, not because we assume they have, without having seen them. Any claim that sources exist must be verifiable. Unless you can indicate what and where the sources are, they are not verifiable.

Keep in mind, as well, that if all you had to do to prevent an article's deletion was to guess at the possibility that better sources might exist than anybody has actually found, then even outright hoaxes would not be deletable anymore, because anybody can say this about literally any article whether its topic actually exists or not. If you are so sure that enough quality sources exist to salvage the article, then find those sources and show them.

Note that sometimes editors posit an even weaker version of this argument: but there may be sources! All criticism of the stronger argument applies in such case too, of course, plus the slippery slope to extreme inclusionism (but there is a tiny chance there are sources... but maybe sources will appear tommmorrow... etc.).

Frequently asked questions
Q. But what if I feel very strongly about this article and sources on the topic are hard to find?

A. The article can be moved under your Wikipedia account where in most cases it can be edited for as long as necessary without fear of deletion. Once those hard to find sources have been located and used to improve the article, an admin can move the article back into the main article space.

Q. But the article is only X days/weeks/months old, references aren't there yet but they will be. Without the Wikipedia article, how will 3rd parties know to write about this?

A. This idea is completely backwards to how Wikipedia actually works. The references must come first, then the Wikipedia article. Wikipedia isn't here to promote ideas to the point where they may become notable, that notability must come first.

Q. But what about WP:BEFORE?

A. Insisting that an article be kept only because the nominator has not followed WP:BEFORE is unhelpful and borders on wikilawyering because it focuses on procedural quibbles instead of addressing the problem (and unsourced articles are a problem). If an article cannot be sourced then it should be deleted and complaining that the nominator hasn't dotted their i's and crossed their t's is not going to change that. The best thing to do is to look for sources; if the nominator has not done due diligence and good references actually prove easy to find, then the deletion will fail and the nominator will be left with egg on their face. However, also note that not everybody has access to the same research tools, so the fact that you were able to access a database that provided more coverage than somebody else found in other databases is not, in and of itself, proof that the other editor was negligent in their duties. If you can salvage the article, then just salvage it and don't attack other editors for not finding what you found.

Q. Why don't you go and look for sources?

A. Frequently people do. This is a collaborative encyclopedia that works on a process of incremental improvement. But demanding people do your work for you is not fair, for several reasons:
 * 1) It's insulting. They are challenging material because they have been unable to find sources. To tell them that they should go away and do just that comes across as a slap in the face.
 * 2) It's presumptuous. You do not have a claim on other editors' time. You are adding or defending material that, as it stands, does not conform to Wikipedia's requirements and it is nobody else's job to fix it. It is unfair to pass this job on to other editors who may not have the time, inclination or knowledge of the subject material to fix it, especially if they believe in good faith that it can't be done at all.
 * 3) Wikipedia policy puts the responsibility on the editor who adds the material to reference it, not the person challenging it.

Q. But what about biting newcomers?

A. We were all newcomers at some point and someone helped us understand Wikipedias policies and guidelines around references. You repay that by doing the same for newcomers and doing it without biting them. Giving new editors who make vague statements presuming existence of sources a "pass" simply because they are new does not do them or Wikipedia any service.