How to build your own investing system

What an investing system is

An investing system is the full set of written rules for how you make every investing decision and constrain your own behavior. It answers three questions: what do I buy (selection), when do I sell (exit), and how much do I buy (sizing). Unlike a "strategy" — a single play — a system integrates many judgments into one repeatable framework.

Most retail investors don't lose because their analysis was wrong. They lose because three weeks later they quietly drop the rules they wrote down. So half a system's value is making you think clearly; the other half is pulling you back when you're about to act on impulse.

Step 1: Define your circle of competence

Your circle of competence is the area you genuinely understand well enough to judge long-term value. It doesn't need to be large, but its edges must be clear:

• List the industries or companies where you can explain "how it makes money" in one sentence;
• Anything you can't — put it outside the circle. Not bad, just not yours;
• Pass on opportunities outside the circle, however tempting. Missing out doesn't lose money; not understanding does.

Step 2: Write a thesis for every position

Before you buy, write down — in your own words — why: the core logic, what you expect to happen, and on what basis. That text is the "constitution" for this investment.

Without a written thesis, you can't judge later whether to sell — because you've forgotten why you bought.

The key is to use your own words and keep the original text. Months later, when the market moves, that original wording is your only reliable anchor.

Step 3: Pre-set your exit conditions

At the time of buying, decide: what would have to happen for me to be wrong and to sell? These are your falsifiers — they turn selling from an emotional call into a rule-based one. Common ones:

• The original core logic is disproven (e.g. the growth story breaks);
• Fundamentals deteriorate materially, outside your plan;
• A clearly better opportunity appears (opportunity cost).

Note: a falling price is not, by itself, an exit condition. If the logic holds and the price drops, that may be a chance to add; if the logic broke, that's the sell signal. Separating the two is the mark of a mature system.

Step 4: Position sizing and risk budget

Even great logic can be wrong, so position size determines how many times you can afford to be wrong. Two minimum rules:

• Per-name cap: no single position exceeds some share of the portfolio (e.g. 10–20%), so one name can't sink you;
• Risk budget: decide up front "the most I can accept losing on this," and size backward from that.

Step 5: Build a review loop

A system isn't built once — it grows out of trading. Periodically revisit your decision records and theses and ask: was the judgment right? Luck or skill? Which rule needs fixing? That's a proper review — it turns every trade into fuel for improving the system.

The hard part isn't building it — it's sticking to it

You'll notice the five steps above aren't hard to think through. What's hard is this: a few weeks later the market moves, and you start chasing, you want to cut, you throw the rules out the window. The real enemy of a system is your own emotion.

That's exactly what Hexis is built to solve: it stores the thesis you wrote in your own words, and the moment you're about to break your rules, it puts your own past words back in front of you and asks one thing: "Is this still consistent with what you said back then?"

FAQ

Disclaimer: This article is for educational discussion of investing methods only. It is not investment advice, a recommendation to buy or sell, or a promise of returns. Examples are illustrative and not about any specific security. Markets carry risk; make your own decisions.