From p123 of "Come Into My Trading Room", Dr Alexander Elder

 

"Beginners become emotional when they trade, but if you want to
survive and succeed, you must develop discipline. The moment
you become aware of feeling fear or joy, use that as a signal to tighten
your discipline and follow your system. You developed that system
when the markets were closed and you felt calm. Now it gives you
your only chance of survival and success in the markets.
The idea of an automatic trading system is fundamentally flawed. If
those systems could work, then the smartest guy with the biggest com-
puter would have cornered the market long ago. Automatic systems do
not work because the market is not a mechanical or electronic entity
that follows the laws of physics. It is a huge crowd of people acting in
accordance with the imperfect laws of mass psychology. Physics and
mathematics can help, but trading decisions must take psychology into
account.
\When you talk with a pro, one of the first questions he'll ask—or
not even ask because he'll know the answer from a few of your com-
ments—is whether you are a discretionary trader or a system trader.
A discretionary trader takes in market information and analyzes it using
several technical tools. He is likely to shift and apply somewhat different
tools to different markets at different times. I-Iis decision-making tree has
many branches, and he follows them at different times as market condi-
tions change. All branches are connected to the sturdy trunk of his deci-
sion-making tree, an inviolate set of rules for risk control.
A system trader develops a mechanical set of rules for entering and
exiting trades. He backtests them and puts them on autopilot. At that
point, an amateur and a pro go in opposite directions. An amateur,
frightened by the market, feels relieved that a system"