How, then, is it possible to decipher an unknown system of writing? Confronted with this primary question we are doubly fortunate in comparison to the decipherers of the Egyptian hieroglyphs more than 150 years ago. In the first place, we have a number of successful decipherments to look back to, both as potential models and as sources of inspiration, which reassure us in the indispensable belief that seemingly impossible feats can be achieved. Yet none of the earlier decipherments is directly comparable to the problem of the Indus script: most of them were based on a translation of a text in the unknown script into a known script and language, or at least the historical context provided crucial clues in the form of proper names. In the absence of such aids, we must look for a different approach.
2. The Indus script and other early writing systems
Here we come to the second advantage we have over Champollion, whose decipherment was greatly hampered by his mistaken preconceptions about the nature of the hieroglyphs. Although the early history of writing is still lacunary, the great lines in the evolution of writing are now fairly well understood. This is important, because it enables us to get an at least approximate answer to one of the two fundamental questions about the Indus script, namely the type of writing represented. For in this case, as elsewhere in scientific research, a big and difficult problem becomes more manageable if it is broken into two or more smaller problems. We shall soon return to the other great unknown, the problem of the underlying language.
The human ability to analyze language and to represent it with written symbols has evolved gradually. In the first stage of "pre-writing", pictures stood for whole sentences or narrations. The next step was to break sentences into separately written words, or rather, morphemes (that is, the smallest meaningful units) which include not only lexical roots but also grammatical markers. In the beginning it was almost exclusively the root morphemes carrying the lexical meaning that were marked in writing, while the grammatical affixes which the root happened to have in any given context were omitted in the script, being left to be guessed and supplied by the reader. This form of writing has been called "nuclear."
Occasionally, however, some basic grammatical affixes were marked even in the earliest script, the archaic Sumerian. Gradually all the grammatical affixes became represented in writing. This complex "logo-syllabic" script demanded many hundreds of graphemes, each of which could have several different meanings. The next major step in the history of writing was the simplification of the system into a "syllabic" script, where only about one hundred signs, each with just one fixed phonetic value, could be used to write about 90% of the texts. The syllabic script was still somewhat clumsy, but eventually, with the successive emergence of the consonantal alphabet and the full alphabet, human speech could accurately and economically be mirrored in writing.
Several criteria taken together enable us to place the Indus script in its proper place within this evolutionary scheme (Figure 2). There are about 450 different signs in the Indus script. Comparing this figure with the number of graphemes in the other early scripts, and also taking into regard the age of the Indus script, the conclusion of its belonging to the logo- syllabic type seems inevitable. It is likely that in most cases the Indus signs stand for nuclear words, i.e. the lexical roots. This is undoubtedly so in the case of texts comprising one single sign only. Important grammatical affixes may also have been marked. Their fundamentally different sign sequences suggest that some Indus texts discovered in West Asia contain local proper names, written in the Indus script with signs having a syllabic value.
[Originally published as Parpola, Asko (1988) Religion reflected in the iconic signs of the Indus script: penetrating into long-forgotten picto+graphic messages. Visible Religion 6: pp. 114-135.]