Our work on building a Latin sense inventory from a small collection of parallel texts in our digital library is based on that of Brown et al. 1991 and Gale et al. 1992, who suggest that one way of objectively detecting the real senses of any given word is to analyze its translations: if a word is translated as two semantically distinct terms in another language, we have prima facie evidence that there is a real sense distinction. So, for example, the Greek word archê may be translated in one context as beginning and in another as empire, corresponding respectively to LSJ definitions I.1 and II.2.

Finding all of the translation equivalents for any given word then becomes a task of aligning the source text with its translations, at the level of individual words. The Perseus Digital Library contains at least one English translation for most of its Latin and Greek prose and poetry source texts. Many of these translations are encoded under the same canonical citation scheme as their source, but must further be aligned at the sentence and word level before individual word translation probabilities can be calculated. The workflow for this process is shown in Figure 2.

Since the XML files of both the source text and its translations are marked up with the same reference points, "chapter 1, section 1" of Tacitus' Annales is automatically aligned with its English translation (step 1). This results (for Latin at least) in aligned chunks of text that are 217 words long. These chunks are then aligned on a sentence level in step 2 using Moore’s Bilingual Sentence Aligner [Moore 2002], which aligns sentences that are 1-1 translations of each other with a very high precision (98.5% for a corpus of 10,000 English-Hindi sentence pairs [Singh and Husain 2005]).

In step 3, we then align these 1-1 sentences using GIZA++ [Och and Ney 2003]. Prior to alignment, all of the tokens in the source text and translation are lemmatized, where each word is replaced with all of the lemmas from which it can be inflected (for example, the Latin word est is replaced with sum1 edo1 and the English word is is replaced with be). This word alignment is performed in both directions in order to discover multi-word expressions (MWEs) in the source language.

Figure 3 shows the result of this word alignment (here with English as the source language). The original, pre-lemmatized Latin is salvum tu me esse cupisti (Cicero, Pro Plancio, chapter 33). The original English is you wished me to be safe. As a result of the lemmatization process, many source words are mapped to multiple words in the target — most often to lemmas which share a common inflection. For instance, during lemmatization, the Latin word esse is replaced with the two lemmas from which it can be derived — sum1 (to be) and edo1 (to eat). If the word alignment process maps the source word be to both of these lemmas in a given sentence (as in Figure 3), the translation probability is divided evenly between them.

From these alignments we can calculate overall translation probabilities, which we currently present as an ordered list, as in Figure 4.

The weighted list of translation equivalents we identify using this technique can provide the foundation for our further lexical work. In the example above, we have induced from our collection of parallel texts that the headword oratio is primarily used with two senses: speech and prayer.

The granularity of the definitions in such a dynamic lexicon cannot approach that of human labor: the Lewis and Short Latin Dictionary, for instance, enumerates fourteen subsenses in varying degrees of granularity, from "speech" to "formal language" to the "power of oratory" and beyond. Our approach, however, does have two clear advantages which complement those of traditional lexica: first, this method allows us to include statistics about actual word usage in the corpus we derive it from. The use of oratio to signify prayer is not common in classical Latin, but since the corpus we induced this inventory from is largely composed of the Vulgate of Jerome, we are also able to mine this use of the word and include it in this list as well. Since the lexicon is dynamic, we can generate a sense inventory for an entire corpus or any part of it — so that if we were interested, for instance, in the use of oratio only until the second century CE, we can exclude the texts of Jerome from our analysis. And since we can run our word alignment at any time, we are always in a position to update the lexicon with the addition of new texts.

Second, our word alignment also maps multi-word expressions, so we can include significant collocations in our lexicon as well. This allows us to provide translation equivalents for idioms and common phrases such as res publica (republic) or gratias ago (to give thanks).

Approaches to word sense disambiguation generally come in three varieties:

Corpus methods (especially supervised methods) generally perform best in the SENSEVAL competitions — at SENSEVAL-3, the best system achieved an accuracy of 72.9% in the English lexical sample task and 65.1% in the English all-words task.[8] Manually annotated corpora, however, are generally cost-prohibitive to create, and this is especially exacerbated with sense-tagged corpora, for which the human inter-annotator agreement is often low.

Since the Perseus Digital Library contains two large monolingual corpora (the canon of Greek and Latin classical texts) and sizable parallel corpora as well, we have investigated using parallel texts for word sense disambiguation. This method uses the same techniques we used to create a sense inventory to disambiguate words in context. After we have a list of possible translation equivalents for a word, we can use the surrounding Latin or Greek context as an indicator for which sense is meant in texts where we have no corresponding translation. There are several techniques available for deciding which sense is most appropriate given the context, and several different measures for what definition of "context" is most appropriate itself. One technique that we have experimented with is a naive Bayesian classifier (following Gale et al. 1992), with context defined as a sentence-level bag of words (all of the words in the sentence containing the word to be disambiguated contribute equally to its disambiguation).

Bayesian classification is most commonly found in spam filtering. A filtering program can decide whether or not any given email message is spam by looking at the words that comprise it and comparing it to other messages that are already known to be spam — some words generally only appear in spam messages (e.g., viagra, refinance, opt-out, shocking), while others only appear in non-spam messages (archê, subcategorization), and some appear equally in both (and, your). By counting each word and the class (spam/not spam) it appears in, we can assign it a probability that it falls into one class or the other.

We can also use this principle to disambiguate word senses by building a classifier for every sense and training it on sentences where we do know the correct sense for a word. Just as a spam filter is trained by a user explicitly labeling a message as spam, this classifier can be trained simply by the presence of an aligned translation.

For instance, the Latin word spiritus has several senses, including spirit and wind. In our texts, when spiritus is translated as wind, it is accompanied by words like mons (mountain), ala (wing) or ventus (wind). When it is translated as spirit, its context has (more naturally) a religious tone, including words such as sanctus (holy) and omnipotens (all-powerful). If we are confronted with an instance of spiritus in a sentence for which we have no translation, we can disambiguate it as either spirit or wind by looking at its context in the original Latin.

Word sense disambiguation will be most helpful for the construction of a lexicon when we are attempting to determine the sense for words in context for the large body of later Latin literature for which there exists no English translation. By training a classifier on texts for which we do have translations, we will be able to determine the sense in texts for which we don’t: if the context of spiritus in a late Latin text includes words such as mons and ala, we can use the probabilities we induced from parallel texts to know with some degree of certainty that it refers to wind rather than spirit. This will enable us to include these later texts in our statistics on a word’s usage, and link these passages to the definition as well.

Two of the features we would like to incorporate into a dynamic lexicon are based on a word’s role in syntax: subcategorization and selectional preference. A verb’s subcategorization frame is the set of possible combinations of surface syntactic arguments it can appear with. In linear, unlabeled phrase structure grammars, these frames take the form of, for example, NP PP (requiring a direct object + prepositional phrase, as in I gave a book to John) or NP NP (requiring two objects, as in I gave John a book). In a labeled dependency grammar, we can express a verb’s subcategorization as a combination of syntactic roles (e.g., OBJ OBJ).

A predicate’s selectional preference specifies the type of argument it generally appears with. The verb to eat, for example, typically requires its object to be a thing that can be eaten and its subject to have animacy, unless used metaphorically. Selectional preference, however, can also be much more detailed, reflecting not only a word class (such as animate or human), but also individual words themselves. For instance, the kind of arguments used with the Latin verb libero (to free) are very different in Cicero and Jerome: Cicero, as an orator of the republic, commonly uses it to speak of liberation from periculum (danger), metus (fear), cura (care) and aes alienum (debt); Jerome, on the other hand, uses it to speak of liberation from a very different set of things, such as manus Aegyptorum (the hand of the Egyptians), os leonis (the mouth of the lion), and mors (death).[9] These are syntactic qualities since each of these arguments bears a direct syntactic relation to their head as much as they hold a semantic place within the underlying argument structure.

In order to extract this kind of subcategorization and selectional information from unstructured text, we first need to impose syntactic order on it. One option for imposing this kind of order is through manual annotation, but this option is not feasible here due to the sheer volume of data involved — even the more resourceful of such endeavors (such as the Penn Treebank [Marcus et al. 1994] or the Prague Dependency Treebank [Hajič 1999]) take years to complete.

A second, more practical option is to assign syntactic structure to a sentence using automatic methods. Great progress has been made in recent years in the area of syntactic parsing, both for phrase structure grammars (Charniak 2000, Collins 1999) and dependency grammars (Nivre et al. 2006, McDonald et al. 2005), with labeled dependency parsing achieving an accuracy rate approaching 90% for English (a high resource, fixed word order language) and 80% for Czech (a relatively free word order language like Latin and Greek). Automatic parsing generally requires the presence of a treebank — a large collection of manually annotated sentences — and a treebank’s size directly correlates with parsing accuracy: the larger the treebank, the better the automatic analysis.

We are currently in the process of creating a treebank for Latin, and have just begun work on a one-million-word treebank of Ancient Greek. Now in version 1.5, the Latin Dependency Treebank[10] is composed of excerpts from eight texts, including Caesar, Cicero, Jerome, Ovid, Petronius, Propertius, Sallust and Vergil. Each sentence in the treebank has been manually annotated so that every word is assigned a syntactic relation, along with the lemma from which it is inflected and its morphological code (a composite of nine different morphological features: part of speech, person, number, tense, mood, voice, gender, case and degree). Based predominantly on the guidelines used for the Prague Dependency Treebank, our annotation style is also influenced by the Latin grammar of Pinkster (1990), and is founded on the principles of dependency grammar [Mel’čuk 1988]. Dependency grammars differ from phrase-structure grammars in that they forego non-terminal phrasal categories and link words themselves to their immediate heads. This is an especially appropriate manner of representation for languages with a free word order (such as Latin and Czech), where the linear order of constituents is broken up with elements of other constituents. A dependency grammar representation, for example, of ista meam norit gloria canitiem Propertius I.8.46 — "that glory would know my old age" — would look like the following:

While this treebank is still in its infancy, we can still use it to  train a parser to parse the volumes of unstructured Latin in our collection.  Our treebank is still too small to achieve state-of-the-art results in parsing but we can still induce valuable lexical information from its output by using a large corpus and simple  hypothesis testing techniques to outweigh the noise of the occasional error [Bamman and Crane 2008]. The key to improving this parsing accuracy is to increase the size of the annotated treebank: the better the parser, the more accurate the syntactic information we can extract from our corpus.

The ability to search a Latin or Greek text by an English translation equivalent is a close approximation to real cross-language information retrieval. Consider scholars researching Roman slavery: they could compare all passages where any number of Latin "slave" words appear, but this would lead to separate searches for servus, serva, ancilla, famulus, famula, minister, ministra, puer, puella etc. (and all of their inflections), plus many other less-common words. By searching for word sense, however, a scholar can simply search for slave and automatically be presented with all of the passages for which this translation equivalent applies. Figure 7 presents a mock-up of what such a service could look like.

Searching by word sense also allows us to investigate problems of changing orthography — both across authors and time: as Latin passes through the Middle Ages, for instance, the spelling of words changes dramatically even while meaning remains the same. So, for example, the diphthong ae is often reduced to e, and prevocalic ti is changed to ci. Even within a given time frame, spelling can vary, especially from poetry to prose. By allowing users to search for a sense rather than a specific word form, we can return all passages containing saeculum, saeclum, seculum and seclum — all valid forms for era. Additionally, we can automate this process to discover common words with multiple orthographic variations, and include these in our dynamic lexicon as well.

The ability to search by a predicate’s selectional preference is also a step toward semantic searching — the ability to search a text based on what it "means." In building the lexicon, we automatically assign an argument structure to all of the verbs. Once this structure is in place, it can stay attached to our texts and thereby be searchable in the future, allowing us to search a text for the subjects and direct objects of any verb. Our scholar researching Roman slavery can use this information to search not only for passages where any slave has been freed (i.e., when any Latin variant of the English translation slave is the direct object of the active form of the verb libero), but also who was doing the freeing (who in such instances is the subject of that verb). This is a powerful resource that can give us much more information about a text than simple search engines currently allow.