Authoritative answer on which boots first – Plugins or Themes?

I can’t seem to find anywhere in the WordPress documentation an authoritative answer on the question on whether a WordPress plugin boots first or whether a WordPress theme boots first.

I need to know the answer because an important development decision I am working on depends on it – as it is related to themes that depend on the presence of certain plug-ins.

And by “authoritative answer”, I mean — is the answer as-it-is just an accident of how WordPress is presently designed – subject to being all up-in-the-wind next version of WordPress — or is it a standard aspect of how WordPress is supposed to work?

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