What is history? How we know the past, era by era

History is the study of the human past, built from the evidence that past left behind: documents, objects, buildings, bones, pictures and recorded memories. It is not the past itself, which is gone, but an account of it that can be checked. Historians ask what happened, why, and how they know, and an account is only as good as the evidence and reasoning behind it.

In short

• The past is everything that happened; history is the part people have worked out from evidence, and argued over.
• Historians test every source: who made it, when, for whom, and what they had reason to say or hide.
• Eras such as "ancient" or "medieval" are labels historians agree on for convenience, and they fit some parts of the world better than others.

Where does the word "history" come from?

The word comes from the Greek historia, which meant "inquiry": finding out by asking and looking. Around 430 BC, Herodotus of Halicarnassus opened his account of the wars between the Greeks and the Persians by calling it, in G. C. Macaulay's translation, "the Showing forth of the Inquiry of Herodotus", written "to the end that neither the deeds of men may be forgotten by lapse of time" and "especially that the causes may be remembered" for which the two sides went to war. That sentence still describes the job: record what happened, and explain why.

Herodotus wasn't alone. In China, Sima Qian finished the Records of the Grand Historian by about 91 BC, a history from legendary rulers to his own day that set the pattern for Chinese official histories for two thousand years. In 1377, the North African scholar Ibn Khaldun wrote the Muqaddimah, which asked why dynasties rise and fall and warned historians against believing reports just because they were repeated. Many societies kept the past in other ways too: in oral traditions, king lists, genealogies and songs passed down by specialists.

How do historians know what happened?

They start from sources and sort them into two kinds. A primary source was made at the time, by someone involved or close to it: a letter, a tax record, a diary, a photograph, a law, a soldier's helmet. A secondary source is a later account built from primary ones, such as a textbook or a historian's book. The U.S. National Archives teaches students to read primary sources in four steps on its document analysis worksheets: meet the document, observe its parts, try to make sense of it, and use it as evidence.

Every source has a point of view, so historians ask what it had reason to say and what it had reason to leave out. A victory inscription written for a king tells you what the king wanted people to believe, which is itself evidence, but not a neutral report of the battle. Roman writers' portraits of Cleopatra tell us a good deal about Roman fears and little that can be trusted about her character. A historian looks for independent sources that agree, notices who is missing from the record, such as the poor, women, enslaved people and the conquered, and looks for evidence they left in other forms.

Where writing is thin or absent, other disciplines fill in. Archaeology studies what people built, used and threw away. Studies of ancient DNA trace how populations moved and mixed. Climate records from ice cores and tree rings show droughts and cold spells that written sources only hint at. History, archaeology and the sciences meet in the questions they share, as the note on how science works describes for evidence and testing generally.

How do we know how old something is?

For written records, dates often come from the text itself, or from events such as eclipses that astronomers can date exactly. For objects, the best-known method is radiocarbon dating, which won Willard Libby the 1960 Nobel Prize in Chemistry.

It works like this. Every living thing takes in carbon, including a tiny, steady share of carbon-14, a radioactive form made in the upper atmosphere. When the plant or animal dies, it stops taking any in, and its carbon-14 decays at a known rate: half of it is gone after about 5,730 years. So the share left is a clock.

• A piece of charcoal from a hearth holds a quarter of the carbon-14 that living wood holds. A quarter is half of a half: two half-lives, or about 2 × 5,730 = 11,460 years.
• A bone with one-eighth left has been through three half-lives: about 17,190 years.
• After about 50,000 years, too little is left to measure, so older finds need other methods.

Those raw figures are then corrected, or "calibrated", because the amount of carbon-14 in the air has varied over time. Scientists calibrate against tree rings, which can be counted year by year back more than 12,000 years, and against other records. That's why a radiocarbon date comes with a range, such as 3350 to 3100 BC, rather than a single year.

What are the main eras of history?

Historians divide the past into periods to make it manageable. The names and boundaries are conventions, not facts about the past, and most were invented for Europe. "Middle Ages", for example, means little in Chinese or West African history, so many historians use broader terms such as "post-classical". The table gives the usual divisions with examples from around the world.

Era	Rough dates	What changed	Examples
Prehistory	From about 300,000 years ago to the first writing	Modern humans spread from Africa across the world; farming began about 12,000 years ago	Cave art, the first villages and cities
The ancient world	About 3300 BC to AD 500	Writing, cities, states, empires and written law	Egypt, Mesopotamia, the Indus valley, Shang and Han China, Greece, [[what-was-the-roman-empire
The post-classical or medieval period	About 500 to 1500	World religions spread; long-distance trade; nomad empires	Tang and Song China, the Abbasid caliphate, the Mali empire, the Mongols, the [[what-was-the-black-death
The early modern period	About 1500 to 1800	Ocean empires and global trade; printing; the scientific revolution	The [[what-was-the-ottoman-empire
The modern period	About 1800 to today	Industry, nation states, mass politics, world wars, decolonization	[[what-caused-world-war-1

The line between prehistory and history is the arrival of writing, which came at very different times. The first writing appeared in Mesopotamia around 3300 BC, as records of goods and rations; writing was invented separately in China and in Mesoamerica later. In some regions, written history begins only when outsiders arrived and wrote about the people living there, which is one reason historians lean on archaeology and oral tradition so heavily for those places. Even the first boundary is a moving one: the Smithsonian's Human Origins Program dates the first Homo sapiens to about 300,000 years ago in Africa, as new finds pushed the date back.

Is history just opinion, then?

No. That historians disagree doesn't mean every account is as good as another. Some things are settled beyond reasonable doubt, such as the Visigoths' sack of Rome in AD 410, or that the Holocaust happened, documented in millions of pages of records and testimony. Arguments happen over causes, weight and meaning: how much one leader mattered, which pressures pushed a country to war, what an event changed.

Still argued

How objective history can be is itself an old debate. In What Is History? (1961), E. H. Carr argued that historians choose which facts matter, so every history reflects the questions of its own time. Geoffrey Elton replied in The Practice of History (1967) that disciplined work with sources can recover what actually happened. Most historians now hold a version of both: evidence limits what can honestly be said, and the questions asked still shape what gets found.

Check yourself

• What is the difference between a primary and a secondary source, and why does every source need questioning?
• Explain, with numbers, how radiocarbon dating turns a quarter of the carbon-14 into an age.
• Why do historians call era names "conventions", and where does "Middle Ages" fit badly?

Sources

• The History of Herodotus, Volume 1 — Herodotus, translated by G. C. Macaulay, Project Gutenberg
• Document analysis worksheets — U.S. National Archives
• The Nobel Prize in Chemistry 1960: Willard F. Libby — Nobel Prize Outreach
• Homo sapiens — Smithsonian National Museum of Natural History, Human Origins Program