Reality

According to the Fourth Doctor, the differentiation of dreams from reality was an age-old conundrum. His own definition of reality did not include that which was only imagined.

Zhuang Zhou, writing in the 4th century BCE, once pondered to himself whether he was a man who had dreamt he was a butterfly, or a butterfly now dreaming that he was a man. (AUDIO: The Crowmarsh Experiment)

Someone else once asserted that there was only one certainty: cogito, ergo sum. Gideon Crane, believing himself to be the Doctor, challenged the real Eighth Doctor's conviction in his reality, saying that he could know no more than this: "I think, therefore I am." (AUDIO: Minuet in Hell)

Accessing the Eighth Doctor's memories, Crane relayed something which had once been said to the Doctor:

The Eighth Doctor, for his part, asserted that his memories must be indicative of reality, and sought to prove this with logical reasoning and evidence. (AUDIO: Minuet in Hell)

When the Sixth Doctor regarded the inside of the Matrix as not taking part in reality, the Valeyard criticised his one-dimensional view toward the concept. (TV: The Ultimate Foe)

According to both the Fourth Doctor and "Dr Stewart", when faced with two options, one ultimately became more certain of whichever reality one wanted more to believe in. For instance, Leela became more convinced of the false reality involving the Crowmarsh Institute after discovering that this world included Marshall, still alive and blissfully married to her. As she began to believe, false memories crept in which seemed to validate the lies she had been told. Jennifer became certain of the real world again when Leela spoke her daughter's name, which she could not possibly have known otherwise, and which also motivated Jennifer to return as she had promised her. (AUDIO: The Crowmarsh Experiment)

On being presented by the Dream Lord with two different dreams, the Eleventh Doctor and Rory Williams each became convinced that the scenario they preferred was reality, and the other a dream. Amy Pond decided that Upper Leadworth was a dream only after losing Rory there, at which point she no longer wanted to believe it could be real. (TV: Amy's Choice)

In some cases, however, reality was differentiated from fantasy, particularly when "cold, hard reality" was one's only option. (TV: The Girl Who Waited) The Twelfth Doctor had trouble distinguishing between the two, as reality and fantasy were "both ridiculous". (TV: Last Christmas)

According to the Third Doctor, the Atlanteans invented legends to "tame" reality when reality became unendurable. Later civilisations came to doubt whether their myths, gods and creatures were "real". The Doctor believed that they were, in part. (TV: The Time Monster)

Whenever Amy Pond as a Ganger saw Kovarian during an adventure, this was, according to the Eleventh Doctor, "reality bleeding through". (TV: A Good Man Goes to War)