Many concepts in Lojban are expressible in two different ways, generally referred to as “afterthought” and “forethought”. Section 4 discussed what is called “afterthought bridi logical connection”. The word “afterthought” is used because the connective cmavo and the second bridi were added, as it were, afterwards and without changing the form of the first bridi. This form might be used by someone who makes a statement and then wishes to add or qualify that statement after it has been completed. Thus,
5.1) la djan. nanmuis a complete bridi, and adding an afterthought connection to make
5.2) la djan. nanmu .ija la djeimyz. ninmu
John is a man or James is a woman (or both)
provides additional information without requiring any change in the form of what has come before; changes which may not be possible or practical, especially in speaking. (The meaning, however, may be changed by the use of a negating connective.) Afterthought connectives make it possible to construct all the important truth-functional relationships in a variety of ways.
In forethought style the speaker decides in advance, before expressing the first bridi, that a logical connection will be expressed. Forethought and afterthought connectives are expressed with separate selma'o. The forethought logical connectives corresponding to afterthought ijeks are geks:
5.3) ga la djan. nanmu gi la djeimyz. ninmu
Either John is a man or James is a woman (or both).
“ga” is the cmavo which represents the A truth function in selma'o GA. The word “gi” does not belong to GA at all, but constitutes its own selma'o: it serves only to separate the two bridi without having any content of its own. The English translation of “ga ... gi” is “either ... or”, but in the English form the truth function is specified both by the word “either” and by the word “or”: not so in Lojban.
Even though two bridi are being connected, geks and giks do not have any “.i” in them. The forethought construct binds up the two bridi into a single sentence as far as the grammar is concerned.
Some more examples of forethought bridi connection are:
5.4) ge la djan. nanmu gi la djeimyz. ninmu
(It is true that) both John is a man and James is a woman.
5.5) gu la djan. nanmu gi la djeimyz. ninmu
It is true that John is a man, whether or not James is a woman.
It is worth emphasizing that Example 5.5 does not assert that James is (or is not) a woman. The “gu” which indicates that “la djeimyz. ninmu” may be true or false is unfortunately rather remote from the bridi thus affected.
Perhaps the most important of the truth functions commonly expressed in forethought is TFTT, which can be paraphrased as “if ... then ... ”:
5.6) ganai la djan. nanmu gi la djeimyz. ninmu
Either John is not a man, or James is a woman.
If John is a man, then James is a woman.
Note the placement of the “nai” in Example 5.6. When added to afterthought selma'o such as JA, a following “nai” negates the second bridi, to which it is adjacent. Since GA cmavo precede the first bridi, a following “nai” negates the first bridi instead.
Why does English insist on forethought in the translation of Example 5.6? Possibly because it would be confusing to seemingly assert a sentence and then make it conditional (which, as the Lojban form shows, involves a negation). Truth functions which involve negating the first sentence may be confusing, even to the Lojbanic understanding, when expressed using afterthought.
It must be reiterated here that not every use of English “if ... then” is properly translated by “.inaja” or “ganai ... gi”; anything with implications of time needs a somewhat different Lojban translation, which will be discussed in Section 18. Causal sentences like “If you feed the pig, then it will grow” are not logical connectives of any type, but rather need a translation using “rinka” as the selbri joining two event abstractions, thus:
5.7) le nu do cidja dunda fi le xarju cu rinka le nu ri ba banro
The event-of (you food-give to the pig) causes the event-of (it will grow).
Causality is discussed in far more detail in Chapter 9.
Example 5.8 and Example 5.9 illustrates a truth function, FTTF, which needs to negate either the first or the second bridi. We already understand how to negate the first bridi:
5.8) gonai la djan. nanmu gi la djeimyz. ninmu
John is-not-a-man if-and-only-if James is-a-woman,
Either John is a man or James is a woman but not both.
How can the second bridi be negated? By adding “-nai” to the “gi”.
5.9) go la djan. nanmu ginai la djeimyz. ninmu
John is-a-man if-and-only-if James is-not-a-woman.
Either John is a man or James is a woman but not both.
A compound cmavo based on “gi” is called a gik; the only giks are “gi” itself and “ginai”.
Further examples:
5.10) ge la djan. nanmu ginai la djeimyz. ninmu
John is-a-man and James is-not-a-woman.
5.11) ganai la djan. nanmu ginai la djeimyz. ninmu
John is-not-a-man or James is-not-a-woman.
The syntax of geks is:
[se] GA [nai]
and of giks (which are not themselves connectives, but part of the machinery of forethought connection) is:
gi [nai]