Is starting your sentence with “Which is why...” grammatically correct?

Question

Is starting your sentence with “Which is why...” grammatically correct?

…our brain is still busy processing all the information coming from the phones. Which is why it is impossible to actually rest this way.

Answer 1

John Lawler: The which in Which is why simply refers to the previous statement. Which is why you can start the next clause this way; it sounds like a new sentence and may contain a pause, and there are no spaces or punctuation in language, i.e, speech.

That's right, of course.

And starting a sentence with it is a stylistic choice, and is not grammatically relevant. That said, it is not a written form. Most editors would cross it out and put it: This is why etc.

Furthermore, in the same vein here is the Master's text (John Lawler, in case you're wondering) on punctuation which states among other things:

[...] punctuation is of recent adoption (Shakespeare’s punctuation, for instance, is quite different from Modern English); it is subject to great individual variation (like handwriting, or spelling before the introduction of printing); and there is no consensus as to how it should be used (though there is no shortage of prescriptivist fulmination, all incomplete and contradictory in one way or another; e.g, Truss 2004). Thus it must also be considered a representational system that is still in the process of being standardized, and may not in fact ever achieve standardization.

[...] Still another use – indeed, the original use – of punctuation is to provide some indication of the intonational and rhythmic intentions of the writer, as the written words indicate the lexical and grammatical intentions.

ERGO, the question about this:
[...]"our brain is still busy processing all the information coming from the phones. Which is why it is impossible to actually rest this way". is about punctuation and not grammar."

No one would dispute this if the "which is why" was placed like this:

[...]"our brain is still busy processing all the information coming from the phones which is why it is impossible to actually rest this way."

The fact is that prosodically, a second sentence could be started by any of the most common relative pronouns: who, whom, whose, which, and that.

Below is a great example of a formal speech by Prodi of the European Commission. It may be a translation, I don't know, but it is nicely done.

The White Paper lays down five principles: openness, participation, accountability, effectiveness and coherence. These principles effectively serve to "apply better the fundamental principles of subsidiarity and proportionality that underlie our Treaties" and they concern all EU institutions.

Which is why we are seeking to:

achieve more transparency in the day-to-day application of EU policies and greater participation by organised civil society and the representatives of municipalities, towns and regions;" [etc.]

European Commission

All this said, when a text is written to be read by readers, most editors would use a period and do this:

"[...]our brain is still busy processing all the information coming from the phones. This is why it is impossible to actually rest this way". is about punctuation and not grammar."

So, really this is an editing question and not a grammar question.

Answer 2

Which (relative pronoun) is anaphoric. It can only refer to a noun that has already been mentioned; otherwise it is meaningless.

In current Modern English, it is expected that the referent noun will be part of the same sentence as "which" and that "which" will introduce a subordinate relative clause.

Subordinate clauses are not themselves sentences, thus your example should be marked as wrong.

However, "Which is why..." is commonly used to complete a statement.

[Two instructors A and B address soldiers]

A: "The bullet comes out the barrel at a speed of a little over 800mph."

B: "Which is why you should not look down a barrel."

or ironically to state the obvious:

A: "So, this key that you gave me will unlock the door to the storeroom?"

B: "Which is why I gave it to you... or did you forget? Or perhaps you were not listening?"

Answer 3

…our brain is still busy processing all the information coming from the phones. Which is why it is impossible to actually rest this way.

It's perfectly OK.

The sequence Which is why it is impossible to actually rest this way is a non-defining relative clause where the antecedent of "which" is the preceding clause our brain .... phones.

Internally, the sequence why it is impossible to actually rest this way is a subordinate interrogative clause (embedded question), so the meaning can be glossed as

"… our brain is still busy processing all the information coming from the phones. Which is the answer to the question 'Why is it impossible to actually rest this way?'"

Answer 4

As usual, the best way to determine if something is generally grammatical is to ask whether it's used in reputable publications. The answer to that question here is a resounding YES.

The TIME magazine corpus yields 70 hits.

Oil, gas and coal leases on federal land require a 12.5% gross royalty, but hard-rock mining pays nothing to the U.S., and a suitability review is an airy dream. Which is why mining-industry money has watered the grass roots of pro-development " wise use " (Mother Lode Vs. Mother Nature, John Skow, 1993)

Critics of Clinton will undoubtedly say that a President with flexible beliefs, who once polled voters to decide where he should go on vacation, deserves history's inattention. Which is why with the end of his presidency in sight and the realization that a lame duck's influence drops precipitously after his sixth year, Clinton and his advisers are feeling the shadow of Reagan and urgently pondering the question, What is Clintonism? (Clinton's Last Campaign; KAREN TUMULTY AND JAY BRANEGAN; 1998)

Like any Yankee or Marlin, you want to shake the stiffness of winter out of your carcass and bound into spring like a line drive. Which is why following a spring-training regimen of muscle-stretching and flexibility exercises can be so important. (Get Spring In Your Training; TIM PADGETT; 2006)

COHA (Corpus of Historical American English) yields 400+ hits.

THERE is a widespread conviction that Bishop William Montgomery Brown really is a heretic. But there is another widespread conviction that heresy hasn't hurt him any. Which is why he stays in the headlines even after his case has been " finally closed ". If heresy had broken out on the old man, or soured his soul or spoiled his looks, the reporters would have noticed it long before this. (New Yorker, 1926)

A good male haircut, it seems, should be a secret that no one's in on except man and his barber. Which is why, as soon as he sits down in the barber chair, he usually warns the barber, " Now go easy? not too short!" (Good Housekeeping, 1946)

We're all part of one great machine. So I feel a little ahead of a lot of people who don't see that yet. Which is why on Sunday morning I leave my pajamas on the bathroom floor and sit on the toilet lid polishing my low shoes with spit and bits of toilet paper. (Harpers, 1968)

Pluralism in Nicaragua would not only be good for Nicaragua, it would be good for all of Central America. It would be the only reliable guarantor of nonintervention. Which is why the proposal made by a group of Nicaraguan rebel leaders meeting in early December with Special Envoy Richard Stone in Panama is a good one. (The New Republic, 1983)

There are, in fact, so many options that the beginner may have trouble selecting the best process for the job at hand. Which is why we selected three different types of home blues for step-by-step demonstrations. (Outdoor Life, 1995)

They create a place -- neither esthetic heaven nor didactic classroom -- where we can begin to experience another way of being human. Which is why we go to the museum in the first place. (NYT, 1996)

This evening the display is washed out and unimpressive, the sun veiled by tight-knotted clouds. The crowd is sparse and mumbly with disappointment. Which is why he's not surprised to find Babs there. He spots her with her chest pressed against the steel railing, breasts spilling down like batter overrunning a pan. (Southern Review, 2002)

The British National Corpus yields 106 hits.

Many of the symptoms shown by food-intolerant and chemical-sensitive patients are symptoms that we all suffer from at times -- headaches, tiredness and indigestion for example. Which is why some doctors feel that such patients are' not really ill', simply over-reacting to everyday symptoms. (The complete guide to food allergy and intolerance. Gamlin, Linda and Brostoff, Jonathan; 1989)

Orchestral players themselves don't necessarily lead the healthiest lives! And yet they are under enormous stress when they are doing their best work. An orchestral player works as hard as a navvy. Which is why I have always tried to insist that my players have proper ways of relaxing away from their work. (Conversations with Karajan. Osborne, Richard. Oxford; 1991)

Price-resistance, payment problems, piracy -- in many parts of the world, no matter how great the aspirations of the local population, plans to supply demand can rapidly turn from opportunity to threat. Which is why so often the thoughts of publishers return to Europe, by which is generally meant the mature markets of southern Europe, Greece, Spain and Italy. (Bookseller. London: J Whitaker & sons, 1993)

The iWeb corpus yields 16501 hits.

It is quite clearly an accepted way to start a sentence in English, and thus grammatical. Any explanation as to why is secondary to the fact that it is in common use in respected publications.

Answer 5

According to other contributors on this site, it's "perfectly okay", but it does need to be pointed out that it is not grammatically correct.

The second "sentence" is a subordinate clause which does not have a main clause, and so is technically a sentence fragment.

If you're down the pub bantering to your mates then I expect few people would care to express their concern on the matter, but in more formal contexts, for example a technical exposition, then it is, IMESVHO, "better" to be as formally grammatically correct as possible, especially if you want learned people to take you seriously.