Traditional grammar makes use of the terms “subject” and “object” to describe the roles of nouns in a sentence. Prototypically a subject does the action; an object has the action done to it, for example:

(1) Lucy reads the book
subject   object

Now this is all very well up to a point but when we want to use “subject” and “object” as general labels referring to the meaning of a noun in relation to an action, we run into problems even within a language like English (for some problems that arise cross-linguistically see my previous post). Consider the following:

(2) the book is read by Lucy

In this (passive) sentence, the book has the same relation to the action described by the verb as before, but it appears as the subject not as the object. In case there’s any doubt about this, consider the following pair of examples:

(3) Lucy loves me
subject   object
(4) I am loved by Lucy

I in (4) behaves exactly like a subject: it is in the nominative case (I and not me) and it triggers agreement with the verb (I am), as well as preceding the verb. Yet its relation to the act of loving is pretty much the same as in (3).

Linguists have dealt with this problem by coming up with the notion of thematic roles, employing labels like AGENT and PATIENT to describe them. Unlike the relations of subject and object, these remain constant whether the sentence is active or passive:

(5) Lucy reads the book
(6) the book is read by Lucy

Lucy is the agent in both sentences; the book is the patient.

Whilst linguists have not yet managed to come to any sort of agreement as what all the different thematic roles actually are, the notion nevertheless helps us in making a number of interesting observations. For example, a lot of verbs denoting changes of state can occur both as intransitives (with only one noun, or “argument”, involved in the action) or as transitives (with two arguments). This is the case for example with freeze (the words in capitals refer again to thematic roles):

(7) Nick froze the ice cream
(8) the ice cream froze

What happened to the ice cream is the same in both instances (it froze), and therefore it seems rational to give it the same thematic role (here labelled EXPERIENCER). In (7), though, the ice cream is an object; in (8) it is the subject. One possible analysis of this is that when the CAUSE argument (e.g. Nick in (7)) – to be understood simply as the argument which causes the change of state described by the verb to occur – is not expressed overtly, an EXPERIENCER is “promoted” into the now-vacant subject position.

This might, in fact, be similar to what we see in the passive. Compare example (4) above with example (9) below, where in the absence of an agent the patient is promoted to subject:

(9) the book is read

As a general rule, we might want to say that all sentences require subjects, and that while these are preferentially agents or causes, they may be patients or experiencers too if no agent or cause is available.

Another thematic role which has been suggested is that of INSTRUMENT. In the following example, this is the role of the knife:

(10) Tiberius sliced the bread with the knife

An instrument, informally speaking, is the thing which the agent uses to effect the action. But instruments can also occur in subject position, e.g.

(11) the knife sliced the bread

the knife here can still be considered an instrument: unlike a typical agent, it isn’t doing the action of its own accord, and we assume there is still some unexpressed agent responsible for the slicing. So we have another type of possible alternation: where an agent is omitted, an instrument may be promoted to subject position in its place.

It’s fascinating that as a result of alternations like this the subject of a verb can actually be associated with multiple possible meanings. To give some more examples, the following show that a subject of break might be associated with at least three different roles:

(12) Wilhelmina broke the window (with the snowball)
(13) the snowball broke the window
(14) the window broke

Equally fascinating is that in some cases these alternations can’t occur. For example, in Imhotep ate the peas with a fork, we have an instrument a fork – but eat can’t take an instrument as its subject like break or slice can: we can’t (generally) say *A fork ate the peas to mean “some unspecified person ate the peas with a fork”.

These possible and impossible alternations seem to suggest a lot about the nature of the lexicon and/or the grammar. They are therefore invaluable tools to linguists seeking to understand better how languages work.