I just noticed that my tree prover fails on this simple formula! This is a good opportunity to rewrite the part of the script that does the proving and implement some shortcuts, and maybe some "loop detection".
I've added keyboard commands to Postbote: If the focus is on the frame with the mail listing, press "R" to refresh, "A" to select all mails, and "G" to quickly change the listing offset (this is for Hermann, who has 1600 mails in his mailbox...).
Suppose some theory T(F) implicitly defines the predicate F. If we want to
apply the Ramsey-Carnap-Lewis account of theoretical expressions, we first
of all have to replace F by an individual constant f, and accordingly
change every occurrance of "Fx" in T by "x has f" etc. The empirical
content of the resulting theory T'(f) can then be captured by something
like its Ramsey sentence 
f T'(f), and the definition of f
by the stipulation that 'f' denote the only x such that T'(x), or nothing
if there is no such (unique) x.
In the previous three entries, I've tried to argue that there are no
genuinely implicit definitions: Whenever a new expression is introduced via
an alleged implicit definition, either there is no question of definition
at all, as in the case of new expressions used as bound variables in
mathematics, or there is an explicit definition nearby.
This latter fact, that sometimes explicit definitions are only
nearby, provides a partial vindication of implicit definitions. For
example, let's assume that folk psychology implicitly defines "pain". But
folk psychology itself is not equivalent to the nearby explicit definition.
To get an explicit definition, we have to turn folk psychology into
something like its Carnap sentence. So the theory itself could be called a
genuinely implicit definition.
I've said that an explicit definition introduces a new expression by
stipulating that it be semantically equivalent to an old expression. If
there are no non-explicit definitions, this means that you can only define
expressions that are in principle redundant. Aren't there counterexamples
to this claim?
Consider the definition of the propositional connectives. We can
explicitly define some of them with the help of others, but what if we want
to define all of them from scratch? The common strategy here is to
recursively provide necessary and sufficient conditions for the truth of a
sentence governed by the connective: A 
B is true iff
A is true and B is true.
Scientific theories are often said to implicitly define their theoretical
terms: phlogiston theory implicitly defines "phlogiston", quantum mechanics
implicitly defines "spin". This is easily extended to non-scientific
theories: ectoplasm theory implicitly defines "ectoplasm", folk psychology implicitly defines "pain".
The first problem from the mathematical case applies here too: Since all
these theories make substantial claims about reality, their truth is not a
matter of stipulation. For example, no stipulation can make phlogiston
theory true. That's why, according to the standard Ramsey-Carnap-Lewis
account, what defines a term (or several terms) t occurring in a theory
T(t) is not really the stipulation of T(t) itself, but rather the
stipulation of something like its 'Carnap sentence' x T(x) 
T(t). All substantial claims in T(t) are here cancelled out by the
antecedent.
I vaguely believe that there are no implicit definitions. So I've decided
to write a couple of entries to defend this belief. The defence may well
lead me to give it up, though. Anyway, here is part 1.
Explicit definitions introduce a new expression by stipulating that it be
in some sense synonymous or semantically equivalent to an old expression.
For ordinary purposes this can be done without the use of semantic
vocabulary by stipulations of the form
This weekend, I've moved into my new
flat, which has both a bath room and a fridge, and also lot's of funny
records from the 1970s.
I often wonder to what extent different theories and approaches in
philosophy of language are conflicting theories about the same matter, or
rather different theories about different matters. For example, some
theories try to describe the cognitive processes involved in human speaking
and understanding; Others try to find systematic rules for how semantic
properties (like truth value or truth conditions) of complex expressions
are determined by semantic properties (like reference or intension) of
their components; Others try to spell out what mental and behavioural
conditions somebody must meet in order to understand an expression (or a
language); Others try to find physical relations that hold between
expression tokens and other things iff these other things are in some
intuitive sense the semantic values of the expression tokens; Others try to
discover social rules that govern linguistic behaviour; and so on. How are
all these projects related to each other?
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