LEC 4
1. Truth-Functionality
Propositional logic 的connectives（连接词）are truth-functional
但是，有时候的描述不是true-functional的，比如："Knowing that", "It is necessary that","it is oblligatory that", "It is always the case that".
So we need something other than propositional logic
2. Language of Modal Logic
Language of Propositional Logic:
φ
::=
p
| ¬
φ
|
φ
∧
φ
Language of Modal Logic:
φ
::=
p
| ¬
φ
|
φ
∧
φ
|
□
φ
Also
♢
φ
abbreviation for
¬
□
¬
φ
.
1. □
The symbol
□
has many different meanings!
Alethic
□
φ
means: “
φ
is
necessarily
true.”
Epistemic
□
φ
means: “I
know that
φ
is true.”
Doxastic
□
φ
means: “I
believe
that
φ
is true.”
Temporal
□
φ
means: “At
every time
in the future,
φ
will be true.”
Deontic
□
φ
means: “
φ
should be
true.”
Legal
□
φ
means: “
φ
is
legally required
to be true.”
2. ♢
The symbol
♢
has many different meanings!
Alethic
♢
φ
means: “
φ
is
possibly
true.”
Epistemic
♢
φ
means: “
as far as I know
,
φ
might be
true.”
Doxastic
♢
φ
means: “I
believe
that
φ
might be
true.”
Temporal
♢
φ
means: “At
some time
in the future,
φ
will be true.”
Deontic
♢
φ
means: “
φ
is allowed to
be true.”
Legal
♢
φ
means: “it is
legal for
φ
to be true.”
3. Meaning and Context
4. Formal Meaning
LEC 5
2. Language of Modal Logic
5. Stacking operators
Keeping in mind : you can use multiple □ and /or ♢ in a single formula, and they stack
Example: □(□p ∨ ♢□q) is a formula of modal logic
1.Alethic
which means: "It is necessary that either p is necessary or q is possible necessary"
2. epistemic
"I know that I either know p or consider it possible that I know q"
Example：
