The words are the same either way. How can what I am thinking decide whether it counts?
The Mishnah Berurah (a central commentary on daily Jewish law, written around 1900) rules that unless you fear your congregation will miss the deadline, you should mean this Shema not to count. That sounds strange. You say every word out loud, and a thought switches them off?
The Mishnah (the core text of the Oral Torah) describes someone reading from a Torah scroll who reaches the Shema passage just when it is time for Shema. "If he directed his heart, he has fulfilled it. If not, he has not." The words and the time are the same. The Gemara's first reaction: this shows that mitzvot need intention.
The Gemara goes on to test that reading, and later authorities stayed divided. The Shulchan Aruch records both views and rules that a mitzvah does need intention. You have to mean to fulfill it.
This Shema is built on that rule. It began as a quotation. Rabbi Yehudah HeChasid did two things to make it count: he added Baruch Shem, and he meant it as his Shema. The Mishnah Berurah gives both points. Without Baruch Shem, the verse looks like someone only reporting what is said. And for a mitzvah from the Torah, he writes, the rule is that intention is needed.
Not everyone wanted to lean on a thought. Rabbi Aharon HaLevi of Barcelona (the Ra'ah, 1200s) held that a congregation does better to fulfill the Torah's Shema inside its blessings. He passed on a fix in his teacher's name: here, say only "Hashem is our God, Hashem is one," without the words Shema Yisrael. Rabbi Yoel Sirkis, the Bach, explains the point: that way it cannot count. The Bach himself wanted Baruch Shem dropped on ordinary days.
The Magen Avraham (a commentary on the Shulchan Aruch by Rabbi Avraham Gombiner, Poland, 1600s) answered that there is no need. Say all of it every day. "There is nothing wrong with it, since he does not intend to fulfill his obligation with it." So the siddur keeps every word, and what counts is left to what you have in mind.
Mishnah Berakhot 2:1; Berakhot 13a,Shulchan Aruch, Orach Chaim 60:4,Tur and Beit Yosef, Orach Chaim 46,Bach on Tur, Orach Chaim 46,Magen Avraham 46:16,Mishnah Berurah 46:31The Shema I know is three paragraphs. How can one line be enough?
The Rema (Rabbi Moshe Isserles, whose notes are printed in the Shulchan Aruch) says that this verse, with Baruch Shem after it, can be a person's Shema for the morning. The Shema in the middle of Shacharit is three paragraphs long. How can six words stand in for all of it?
Sefer HaChinukh (a book from Spain in the 1200s that explains each mitzvah) states the Torah's command like this: to read one verse, evening and morning, and the verse is Shema Yisrael. The Sages, he writes, then required us to read three paragraphs.
The Gemara gives that verse a rule of its own. Rabbi Meir taught: "Up to here it needs the intention of the heart." Rava ruled like him, and so does the Shulchan Aruch. Someone who read Shema without directing his heart in the first verse has not fulfilled it. The rest counts even if his mind was elsewhere.
People in the Gemara lived by this. Rav Nachman told his servant Daru: for the first verse, bother me. For more than that, do not. Rashi (the classic commentator, who lived in France and died in 1105) explains: if you see me dozing, bother me until I am wide awake for the first verse.
It was never fully agreed. The people closest to Rabbi Yehudah HaNasi argued about him. Did he go back later and finish the rest of Shema? Bar Kappara said no. Rabbi Shimon, his own son, said yes. Later authorities argue about a related question: is the Torah's obligation one verse, the first paragraph, or more? The Halacha section has what they say to do about it.
What is the attention for? Sefer HaChinukh says the point of Shema is to accept that God is one and is King, and to take to heart that His eyes are open on everything a person does. Then he explains why the Sages insisted on intention here: a person is not reminded of anything unless he puts his mind on it.
So the minimum is short. It is not automatic.
Rema, Orach Chaim 46:9,Sefer HaChinukh 420,Berakhot 13b with Rashi,Shulchan Aruch, Orach Chaim 60:5,Shulchan Aruch HaRav, Orach Chaim 46:9Why does Shema have a deadline? Does God care what time it is?
This whole page exists because of a clock. People were worried enough about saying Shema late that they turned this verse into a backup. Why should the time matter that much?
The time is written into the paragraph itself: speak these words "when you lie down and when you get up." Sefer HaChinukh notices that the two halves work differently. Some people lie down as soon as night starts, some at midnight, some at the end of the night. So the night Shema has the whole night. Getting up is different. No healthy person, he writes, gets out of bed at the end of the day, or even in the middle of it.
Then he gives a reason. A person is physical, he says, drawn after the nonsense of the world and pulled by what he wants. He needs a steady reminder of God's kingship to guard him from sin. So God gave two fixed times. The morning one is "to help with everything we do that day." Someone who remembers in the morning that God is one and is King will have that thought, and the words he said, guarding him all day. The night Shema does the same for the night.
The Chinukh does not spell out the next step, but it follows from his reason. A reminder meant to shape the day belongs near the start of it.
What if you miss the time? The Mishnah says: "One who reads from then on has not lost. He is like a person reading the Torah." The Shulchan Aruch rules that Shema can still be said with its blessings for one more hour, and adds: "He does not have the reward of one who reads it on time." The words are still Torah. What is gone is the mitzvah in its time.
At the other end of the morning are the people the Shulchan Aruch calls vatikin, which Rashi explains as humble people who love the mitzvot. Their way, it says, is the best way to do this mitzvah. They timed Shema and its blessings to finish at sunrise.
Devarim 6:7,Sefer HaChinukh 420,Mishnah Berakhot 1:2,Shulchan Aruch, Orach Chaim 58:1 and 58:6