Databases Reference
In-Depth Information
describes.
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An assertion is either made or not made. But that is
not a relationship between a statement and an object. It is a rela-
tionship between a statement and the person who does or does
not assert it.
We sometimes say, in rough equivalence, that we believe or
do not believe that a statement is true. But just as assertions
are not statements, beliefs are neither statements nor assertions.
Beliefs are what philosophers call
propositional attitudes
. In fact,
assent, assert, claim and say are all speech acts; they are things
we
do
with words. But believe, know and think are propositional
attitudes; they are cognitive stances we take with respect to
those words. (Accept and agree could be one or the other,
depending on whether they refer to behavior or to a behavioral
disposition.)
Assertions, Statements and Time
Conventional tables are the bread and butter of IT. The data
in those tables represent both what things are currently like
and also what we currently believe those things are like. They
represent both what things are like now and what we now
believe they are like.
There is a timeline along which persistent objects are located,
and a timeline along which we hold various beliefs. Data in con-
ventional tables is “pinned”, along both timelines, to the moving
point in time we call “the present” and which, in this topic, we
designate as
Now()
. The maintenance of conventional data is
an ongoing effort to keep up with the changes that follow in
the trail of that moving point.
But as well as the present, there are the past and the future.
So if we “unpin” data along both these timelines, we end up with
nine possible ways that data and time may be related.
In this section, we will use the terminology of beliefs even
though, as we said previously, the nine different terms we
listed there are equivalent, as far as our discussions in this topic
are concerned. This chapter is about assertions, and so we ini-
tially tried to write this section using that terminology. But it
seems to us that the argument is easier to follow using the lan-
guage of beliefs. Nonetheless, we are speaking about assertions,
albeit in the more colloquial language of beliefs. Not all
assertions, of course; and not all beliefs. Rather, as we said ear-
lier, assertions that statements made by rows in database tables
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Assuming, that is, a pre-critical correspondence theory of truth which, for purposes of
clarifying the semantics of bi-temporal data, seems to us perfectly adequate.
