Our sensory organs (which we have established have
limitations) gather information from stimuli in the
environment such as light or sound waves.
Our body converts the stimuli into neural
signals that travel through our nerve cells to
our brains. (Transduction)
Our brains interpret the signals and create
meaning. Sometimes, the complexity of the
signals means it is difﬁcult for our brains to
perceive with full accuracy.
Finally…each person has a different set of eyes and a different
brain. If our brains are perceiving reality, and we each have
different brains, how similar are our realities, really?
“The world or the state of things as they actually exist,
as opposed to an idealistic or notional idea of them.”
How do we each form an
understanding of reality?
Where does our understanding of the world come from?
1. Personal Experience: our ﬁrst-hand sensations and
experiences of the environment.
2. Authorities: certain people are culturally trusted: parents,
teachers, highly educated professionals, and government
3. Factual Evidence: has it been studied, tested, published, and
declared to be a fact?
4. References: can you read about it in books that society
considers to be reliable?
5. Culture/Tradition: This category is related to authority.
Our cultural traditions and religious beliefs can inﬂuence
what we believe to be true.
Is The Earth Flat?
Until sometime near the end of the 15th century (Columbus
sailed in 1492), the vast majority of people believed the
Earth was ﬂat.This idea formed a common world view and
was supported by reasoning and evidence.
• Authorities: Teachers taught that the world was ﬂat.
• References: Some scientiﬁc experts wrote opinions
supporting the belief that the world was ﬂat. (Others,
like Galileo, opposed this belief, but were silenced by
the Catholic Church).
• Factual Evidence: No one had circumnavigated the
earth. (Ferdinand Magellan sailed around the world
• Personal Experience: From a normal vantage point,
the earth looks ﬂat.
Is The Earth Flat?
Today, we believe that the world is round, and just like the
ancients, we base this belief on facts and experience.
Every year, humanity discovers new things. How different will our
collective understanding of reality look 100 years from now?
• Authorities: My parents and teachers all agree that
the earth is round.
• References: I read about this in science textbooks
and I saw it in reputable TV documentaries about the
planet and the solar system.
• Factual Evidence: I have seen photographs taken
from outer space that show that the earth is round.
• Personal Experience: I have ﬂown across the world
in an airplane and have seen the curved horizon line of
“Multiple perspectives” means that we each experience
reality from different points of view: through the lens of
different cultures, histories, religions, families, and schools.
All disagreement in the world is the result of seeing the
world (including events, ideas) from different perspectives.
How can we use our understanding of multiple
perspectives to explain human behavior?
History’s Great Philosophers
tackle the question of reality
Plato’s Allegory of the Cave
Plato’s famous allegory is
about the perception of a
limited reality. Imagine
prisoners trapped in a cave
for their entire lives. In the
cave they can see shadows,
but not the source of the
shadows. Because they have
no knowledge of ﬁre or of
the objects casting the
shadows, the shapes they
see on the wall and the
echoes they hear are the
only experiences they have
to create an understanding
of reality with.
Rene Descartes’ Evil Demon
In the 17th century, Descartes came up
with a thought experiment: he believed
that God created the world…but what if
everything we see and experience is a
vision created by -not a god- but an all-
powerful, evil demon? He thought,“what
if the demon is controlling my mind,
and creating everything I experience?”
…the “brain in a jar.”
Rene Descartes’ Evil Demon
Descartes tried to think
of something an evil
demon could never
deceive him about. He
decided that he cannot
be deceived into
thinking that he does
not exist, because if he is
thinking, he must be
existing. One must exist
in order to think, so if
you are thinking, you
know for certain that
Hume’s Idea of Causality
Hume was very different than Descartes: Descartes believed that our
minds are the basis of truth, while Hume believed that experience is the
basis of truth. He asked,“if something happens repeatedly (like the rising
of the sun), does it mean it is true that it will always rise?” He concluded
that past experiences do not predict the future: our belief that the future
will resemble the past is a matter of habit.
We cannot imagine things we
have never experienced! Every
idea we create is a combination
of things we have seen. Also…
Facts can never be proven from
secondhand knowledge: they
must be discovered or inferred
from personal experience.
The “egocentric predicament” is the problem of not
being able to view reality outside of our own
perceptions. (I don't know anything about the world,
other than what is given to me).
We are like the prisoners in
Plato’s Allegory of the Cave:
the only reality we know is the
one based on our experiences
(despite the fact that there
could be a different reality
beyond what we are able to
Charles Sanders Pierce came up with the term
“phaneron” to describe reality as it is ﬁltered through
our perceptions. He said that reality as we see it
and true reality are two separate things.
can't prove that
in the end, all of
we acquire is the
product of our
Solipsism & Realism
A solipsist is someone who believes
that everything we experience is a
product of our minds.Thus, if I cease
to exist, everything else ceases to
exist as well.
A realist is someone who believes
that reality exists separately from our
minds. Thus, if I cease to exist, the
outside world continues existing
However, it is impossible to prove that reality exists separately from
our minds! We just believe it to be true.
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