\subsection{Dreaming}
‘I see plainly that there are never any sure signs by means of which
being awake can be distinguished from being asleep’ (Meditation I)
‘... the principal reason for doubt, namely my inability to distinguish
between being asleep and being awake.
For ... there is a vast difference between the two, in that dreams are
never linked by memory with all the other actions of life’ (Meditation 6)
‘when I distinctly see where things come from and where and
when they come to me, and when I can connect my perceptions of them with
the whole of the rest of my life without a break,
then I am quite certain that when I encounter these things
I am not asleep but awake’ (Meditation 6).
Do any considerations about dreaming provide
‘reasons ... which give us possible grounds for doubt about all things, especially material things, so long as we have no foundation for the sciences other than those we have had up until now’?
\subsection{Cosmic deception}
‘How do I know that he has not brought it about that there is no earth,
no sky, no extended thing, no shape, no size, no place, while at the same
time ensuring that all these things appear to me to exist just as they do now?
What is more, since I sometimes believe that others go astray in cases
where they think they have the most perfect knowledge,
may I not similarly go wrong every time I add two and three or count the
sides of a square, or in some even simpler matter, if that is imaginable?’
Is this a reason to doubt all things?
The usefulness of extensive doubt ‘lies in freeing us from our preconceived opinions,
and providing the easiest route by which the mind may be led away from the senses.’