David K.H. Begg, Gianluigi Vernasca-Economics-McGraw Hill Higher Education (2011)

CHAPTER 5 Consumer choice and demand decisions
U'
If indifference curves intersected, the consumer
would be indifferent between X and Yon the
indifference curve UU, and between Y and Z
on U'U', and hence indifferent between X and
Z. Since Z ofters more of both goods than X, this
violates the assumption that consumers prefer
more to less. Indifference curves cannot intersect.
Meals
Figure 5.4
Indifference curves cannot intersect
Furthermore it should be noticed that, if indifference curves cross, the assumption regarding the transitivity
of tastes will not hold. If X is indifferent to Y and Y is indifferent to Z, then by the logical consistency of
tastes we should have that Xis indifferent to Z. However, we know that Z should be preferred to X because
of the assumption that consumers prefer more to less.
Our assumptions about consumer tastes rule out intersecting indifference curves.
Other contour maps
II
When you look at a good map, you will see concentric rings or contours, each showing points of
equal height. They are like indifference curves but do not have to obey the law of diminishing
marginal rate of substitution and hence have stranger shapes. But they never intersect. Different contours are
different heights.
As you rise through successive contours, you reach
a dot marking the top of the mountain. In economics,
we hardly ever reach the top. People are rarely
satiated. But an indifference map for champagne and
lobster might look like a mountain. Too much of
either, and you are sick. The dot for the absolute
best, or bliss, point, equivalent to the top of the
mountain, then shows the finite combination of
champagne and lobster preferred above all others,
however much is available. We are now violating our
assumption that the consumer always prefers more
to less. The part of the contour sloping up corresponds
to the range in which more is no longer better.
For broad categories of commodities, we are never
satiated. It is as if we are confined to the shaded area
of the diagram.
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Quantity of good H
Bliss
96