Investors can forget the “death cross,” “bearish divergences” and “symmetrical triangles,” and what the Federal Reserve says it will do about interest rates, and just focus on Occam’s razor: The S&P 500 abandoned its long-term uptrend in late August, meaning it is now in a downtrend.

Occam’s razor is the philosophical principle that suggests, all things being equal, the simplest explanation tends to be the right one.

One of the most elementary trading maxims on Wall Street is “the trend is your friend.” That’s basically what all the short-term technical patterns, economic data and earnings reports are used for, to determine which direction the longer-term trend is heading, and whether it’s about to change.

Read more about death crosses, bearish divergences and symmetrical triangles.

Once that trend is determined, a tenet of the century-old Dow Theory of market analysis says it is assumed to remain in effect, until it gives definite signals that it has reversed, according to the Market Technicians Associations knowledge base. In other words, the trend is your friend, until it isn’t.

After cutting through all the noise, a trendline is probably the best chart pattern to determine the trend, as it is also the simplest. And the simplest way to tell if a trend has reversed, is if the trendline breaks.

Based on Occam’s razor, the four-year uptrend is over

The S&P 500 SPX, +1.05% had been riding a strong weekly uptrend, defined by the trendline connecting the bottom of the last correction in October 2011 with the bottom of the November 2012 pullback and the October 2014 low. The S&P 500 fell below that line in late August, meaning the uptrend flipped to a downtrend.

Based on the Occam’s razor principle, the uptrend was the friend of investors for four years, but now it isn’t.