Whenever one thinks about one’s occurrent or recollected thoughts about something — for example, wondering why one decides on one strategy to accomplish a goal or task, as opposed to some other strategy, or wondering why one cannot recall the name of the person one just ran into — one performs an act of metacognition, one has thoughts about one’s own thoughts or cognitions. One thereby 'metacognizes' or 'metathinks'. Typically we little realize our doing it, yet we metacognize frequently, though often not systematically or programmatically. 
Neither of the principal two components of metacognition — monitoring one's thoughts and controlling them — necessarily require conscious mental processing, though they can and often do. One metacognizes also when one thinks about the very nature of one's thinking processes. The 17th century philosopher, René Descartes metacognized when he declared, "I think, therefore I am" — "Cogito, ergo sum" — and his essay, Rules for the Direction of the Mind (Regulae ad Directionem Ingenii), typifies his consummate skill in metacognition.
Scientist and neuroscience-writer, Jonah Lehrer, gives this succinct description of metacognition:
...an essential feature of the human mind, which is that it doesn't just think. It constantly thinks about how it thinks. We're insufferably self-aware, like some post-modern novel, so that the brain can't go for more than a few seconds before it starts calling attention to itself, reflecting on its own contents, thoughts, and feelings. This even applies to thoughts we're trying to avoid... The technical term for this is "metacognition," and it's a rather surreal skill.
Metacognition does not subsume a unitary concept
- One can metacognize about one’s knowledge or beliefs recalled from longterm memory, whether we remember correctly or incorrectly. One can respond on the witness stand, "I'll never forget that face", then point to the lawyer's assistant instead of the defendant.
- One can metacognize by monitoring one’s present thinking, trying to decide, say, whether one has considered all factors relevant to the thought task at hand: “Let’s see; what have I forgotten?”
- One can metacognize by controlling, or in some way, regulating one’s thinking: “I’m not going to pursue this line of thought, as it seems to lead nowhere, or I’m finding it depressing, or I’ll have to talk to so-and-so about this idea.”
- One may metacognize for purposes of strategizing future activities: “Okay, here’s my plan, but is it a good one.?”
- One can metacognize to try to determine why one has a certain world view: “Why do I think of myself as a slave; aren’t I as much a human being as my master?”
- One can metacognize during learning exercises: "Do I know the material, or do I need to study more?"
- One can metacognize about what one knows about about one's cognitive processes and how one can use that knowledge or improve upon it. One can ask such questions as: "How mindfully do perform my daily activities?"; "What biases do I have when I assert a proposition or interpret what I have seen or heard?"; "How can I improve my memory for peoples' names?"
One can extend this list, which serves only to demonstrate that different categories of metacognition exist, and that the activity of metacognizing occurs as a natural cognitive phenomenon, considered by some scholars a conscious phenomenon, but not necessarily according to others. 
Metacognition and learning
Social scientists, cognitive scientists, psychologists and educators study the nature of metacognition in relation to the learning process.
Metacognition refers to higher order thinking which involves active control over the cognitive processes engaged in learning. Activities such as planning how to approach a given learning task, monitoring comprehension, and evaluating progress toward the completion of a task are metacognitive in nature. Because metacognition plays a critical role in successful learning, it is important to study metacognitive activity and development to determine how students can be taught to better apply their cognitive resources through metacognitive control. 
- Dunlosky J, Metcalfe J. Metacognition. Sage:Los Angeles. ISBN 978-1-4129-3972-0.
- First paragraph of book's Introduction: Metacognition refers to thoughts about one’s own thoughts and cognitions (Flavell, 1979). Although the term itself may seem mysterious, metacognitive acts are common. For instance, take some time to answer two questions. First, when was the last time you failed to recall someone’s name, but were absolutely sure you knew the name? These frustrating events, called tip-of-the-tongue states, happen a lot and may increase in frequency as we grow older (Schwartz, 2002). They are metacognitive in nature because you are having a thought (“I’m sure I know the person’s name”) about a cognition (in this case, your thought is “that the person’s name is in your memory”). Second, when was the last time you decided to write down lengthy directions, or perhaps even brief ones, and how often do you make a list of groceries to buy at the market? In such circumstances, you may realize that there is little chance of remembering important information, so you naturally rely on external aids—for example, lists, PalmPilots, or even other people— to ensure that you won’t forget. Understanding the limits of your own memory also is a form of metacognition because it concerns your beliefs and knowledge about memory. What may also be evident from the rather common events illustrated above is that metacognition is not a single concept, but it is multifaceted in nature.
- Melanie Cary, Lynne M Reder. (2002) Metacognition in Strategy Selection: Giving Consciousness Too Much Credit.] Chapter 1. In: Patrick Chambres, Marie Izaute, Pierre-Jean Marescaux (editors).Metacognition: Process, Function, and Use. Springer. ISBN 1402071345, ISBN 9781402071348.
- Chapter Abstract:Many researchers believe that metacognitive processes regulate strategy selection. Another common assumption is that metacognitive processes, such as strategy selection, entail conscious processing or decision making....[We] examine whether conscious awareness is a critical aspect of strategy selection. We review evidence that first establishes that strategy selection varies both across and within individuals in response to dynamic features of the environment. Then, we present evidence that strategy adaptation can occur without (a) conscious consideration of different strategies or (b) conscious awareness of factors influencing one's strategy use. Specifically, shifts in strategy use occurred when people seemed to be unaware (a) that there were shifts in their strategy use or (b) that there were changes in the characteristics of the environment that, nonetheless, affected their strategy use..
- Reder LM, Schum CD. (1996) Chapter 3: “Metacognition Does Not Imply Awareness: Strategy Choice Is Governed by Implicit Learning and Memory.” In: Reder LM (editor) Implicit Memory and Metacognition. Lawrence Erlbaum:Mahwah, NJ.
- Excerpt: ....there are two core meanings of the term metacognition to which most researchers using that label often refer: monitoring and control of cognitive processes....Monitoring of cognitive processes can include awareness of the component steps in cognitive processes as well as awareness of various features of these steps including their duration and their successfulness....Monitoring typically refers to awareness of the features of the current behavior....In contrast, control of cognitive processes refers to the processes that modify behavior, such as the selection of a strategy for performing a task....we focus on the relationship between monitoring and control of cognition in a special way: We argue that some aspects of metacognition typically called monitoring, and therefore implying awareness, actually operate without much awareness. Moreover, the control processes that operate to affect strategy choice are frequently influenced by implicit [non-conscious] processes.
- René Descartes, Roger Ariew. (2000) Philosophical essays and correspondence. Editor: Roger Ariew. Hackett Publishing. ISBN 0872205029. Pages 2-28: "Rules for the Direction of the Mind (Regulae ad directionem ingenii).
- Lehrer J. (2009) Thinking Meta: Good judgment is more than a matter of "gut feeling" or rational deliberation — it´s a willingness to reflect on the decision making process itself. SEED Magazine, February 2009, pp. 58-60. About the author: I'm an Editor at Large for Seed Magazine and the author of How We Decide and Proust Was a Neuroscientist. I graduated from Columbia University and studied at Oxford University as a Rhodes Scholar. I've written for The New Yorker, Nature, Wired, The Washington Post and The Boston Globe. I'm also a Contributing Editor at Scientific American Mind and National Public Radio's Radio Lab.
- Metacognition: Process, Function, and Use. By Patrick Chambres, Marie Izaute, Pierre-Jean Marescaux. Springer, 2002. ISBN 1402071345, ISBN 9781402071348.
- Livingston JA. (1997) Metacognition: An overview.