Accounting begins with evidence, not opinion
You can already write a clean journal entry — date on top, debits first, credits indented, the two columns equal. But step back and ask a question the previous rung quietly skipped: how did you *know* a transaction happened in the first place, and for how much? An entry does not invent itself out of thin air. Something concrete and external must say *this occurred* before a bookkeeper is allowed to touch the books. That something is a source document, and it is where the whole accounting cycle truly begins.
Why insist on this? Because accounting's whole claim to usefulness is that the numbers are *trustworthy* — that they describe real events, not wishful thinking. Recall the monetary-unit assumption: we only record things we can express in money. A source document is what pins the amount down. It answers the four questions every entry needs settled — what happened, when, with whom, and for how much — and it answers them in a form someone *else* produced or signed, so the number is not just the bookkeeper's say-so. Remove the documents and accounting collapses into a story anyone could rewrite.
A field guide to the raw material
Source documents come in families, and learning to recognize them is half the job. An invoice is a seller's demand for payment — a bill — listing what was sold, the quantity, the price, and the date. The same piece of paper is a *sales invoice* to the seller and a *purchase invoice* to the buyer; one event, two filing cabinets. An invoice is the usual trigger for recording a credit sale or a credit purchase, where goods change hands now but cash settles later, creating an account receivable for the seller and an account payable for the buyer.
A receipt proves cash actually moved — it is the acknowledgement that payment was made and received. The distinction between an invoice and a receipt is one beginners blur but accountants never do: an invoice says *you owe me*; a receipt says *you have paid me*. Notice that this is exactly the gap between the accrual and cash basis made physical. Under accrual accounting the invoice may already create revenue; the receipt only later records the cash. Two documents, two different moments, and confusing them is one root of the famous truth that profit is not cash.
The rest of the family rounds out the picture. A *purchase order* is the buyer's formal request to buy — it precedes the invoice and lets a company prove an order was authorized before money was committed. A *bank statement*, issued by the bank, is the independent record of every deposit and withdrawal; because it comes from an outside party, it is prized evidence and the basis for reconciling the company's own cash records against reality. *Payroll records* — timesheets, pay registers, tax withholding slips — document wages earned and deductions, the raw material behind every payroll liability. And a voucher is the internal packet that bundles a document with its approvals, a voucher saying *we checked this and authorized payment*.
The audit trail: why documents are kept
A source document does not retire once its entry is written; it is filed, and the filing is the point. Together, all those filed documents form the audit trail — an unbroken chain that lets anyone walk a number in either direction. From a single line in the financial statements you can trace *back* through the trial balance, the ledger, the journal, and finally to the original receipt or invoice that started it. Or you can walk *forward*: pick any receipt from the drawer and follow it all the way into the statements. When that chain holds at every link, the books are not merely plausible — they are *checkable*.
This is why an auditor's first instinct is to ask, "show me the document." In the language of the audit profession, the source document is the primary piece of audit evidence; documents from outside the company, like that bank statement, are trusted more than ones the company made itself, precisely because the company cannot quietly rewrite them. The trail is also a guardrail against fraud. When every payment must carry an approved voucher, and the person who approves cannot be the same person who records or pays — the principle of segregation of duties — it becomes much harder for any one individual to invent a transaction and cover it up.
What "analyze the transaction" actually means
Every textbook lists "analyze the transaction" as the first step of the cycle, then races on as if it were obvious. It is not obvious, and it is the step beginners most often perform on autopilot. To analyze a transaction is to read a source document and extract from it everything the books need: the date, the amount, and — the hard part — which accounts are affected and in which direction. This is transaction analysis, the very same four-question discipline you already practiced when journalizing, now anchored to a physical piece of evidence rather than a word problem.
Make it concrete. A supplier's invoice lands on the desk: 800 dollars of office supplies, delivered today, payment due in thirty days. You analyze. *Which accounts?* Office Supplies and Accounts Payable. *What type?* An asset and a liability. *Up or down?* Both up — you have more supplies, and you now owe the supplier. *Debit or credit?* An asset rising is a debit; a liability rising is a credit. So: debit Office Supplies 800, credit Accounts Payable 800. The crucial thing is that no cash moved today, because the document said *due in thirty days* — read it carelessly as a cash purchase and you would credit the wrong account and break the audit trail.
SOURCE DOCUMENT ANALYSIS ENTRY ------------------------------------------------------------- Supplier invoice --> Office Supplies (asset, up) Dr 800 $800, due in 30 days Accounts Payable (liab, up) Cr 800 ------------------------------------------------------------- the evidence the thinking the record
Honest cautions, and the road ahead
A few honest warnings keep this from feeling deceptively easy. First, a document existing does not make the entry correct — you can hold a perfectly real invoice and still post it to the wrong account or in the wrong month. The document fixes *what* and *how much*; your analysis still has to get the accounts right. Second, the *amount* on a document is not always the amount you record: an invoice may quote a list price, but discounts, taxes, and freight can change what actually hits the books. Reading a document means reading all of it, not just the big number.
Third, not every entry has a tidy external paper. Some legitimate transactions — recording depreciation, accruing interest that has built up but not yet been billed — have no incoming invoice at all. These are supported instead by an internal memorandum or a calculation schedule, which is itself a source document, just a home-made one. That is why "no document, no entry" is a starting rule and not an absolute law: the deeper principle is that *every entry must be supportable*, by something a reviewer can examine, even if that something is a spreadsheet the company built. Keep that distinction and you will not be thrown when adjusting entries arrive later in this rung.
So here is the whole arc in one breath. A real event produces a document; you analyze the document into accounts and directions; that analysis becomes a journal entry; and the document is filed to anchor the audit trail. Everything still to come in this rung — journalizing, posting to the ledger, and the trial balance — is just this same evidence flowing downstream, growing into statements without ever losing its tether to the original receipt. Master the start and the rest of the cycle is bookkeeping you can trust, because you can always trace it home.