Characteristics of Theoretical Linguistics

The answer to this question very much depends on who's asking and why. And what stage of your linguistic studies you are at.

For the lay person, I'd start with James McCawley's "To ask a professional linguist how many languages they speak is just like asking a doctor how many diseases they have." This pointing to the dual meaning of the word 'linguist' in English but also to the fact that theoretical linguistics is concerned with general principles common to all languages.

It might also be useful to distinguish linguistics from philology - the latter dealing with a description of specific languages and from pedagogic or prescriptive grammar.

The reading I might recommend at this stage is Stephen Pinker's Language Instinct (although I completely disagree with his conclusions, it is a good initial overview).

The definition I would have parroted back to a professor during my undergraduate studies would have been something like: "Theoretical linguistics is characterized by a concern with general principles governing how language works using methods including corpus analysis, contrastive analysis, psycholinguistic experiments, etc.)."

For readings at this stage, I would go to the classics:

• Ferdinand de Saussure: Course in General Linguistics
• Otto Jespersen: Philosophy of Grammar
• Edward Sapir: Language
• Leonard Bloomfield: Language

And if you wanted to get more in-depth:

• Roman Jakobson and Morris Halle: Fundamentals of Language
• Luis Hjelmslev: Prolegomena to a Theory of Language
• Noam Chomsky: Cartesian Linguistics
• MAK Halliday: Explorations in the Functions of Language

But today, I would be more interested in discussing with how various linguists' theoretical preoccupations direct what they study and how. For instance, in order to understand the divide between the Chomskean tradition and the rest of linguistics, theoretical concerns are paramount. Like many social sciences, linguistics has a bit of a science envy, which can lead to overcompensating with formal approaches that obscure more than they reveal.

Some authors who addressed this (off the top of my head):

• Robin Tolmach Lakoff: Language War
• Frederick Newmeyer: The Politics of Linguistics
• George Lakoff: Women, Fire and Dangerous Things (first 3 chapters)
• William Croft: Radical Construction Grammar (first chapter)
• William Labov: Principles of Linguistic Change (vol 1. first few chapters)

There are, of course, many more things to read and many more perspectives to consider. This is not just something that can be answered without going into the very foundations of the study of language and that can't (and shouldn't) be done comprehensively on SE. However, we can probably provide a relatively safe starting point.