Act III, Scene ii of Shakespeare's The Merchant of Venice is very revealing both in terms of plot and in theme. In this scene, Bassanio makes his choice of casket in pursuit of winning Portia's hand in marriage. Portia tries to delay his choice, fearing losing his company, but the more Portia speaks of her love for him, the more Bassanio feels that delaying his choice is only torturing him. Bassanio makes the speech in question while choosing his casket. In this speech, he especially reflects on how he will not choose based on the outward appearances of the caskets because he knows very well just how deceptive outward appearances can be. In fact, he says that far too often something beautiful actually encloses something evil. The particular lines in question further illustrate his thoughts concerning deceptive exteriors:
There is no vice so simple, but assumes
Some mark of virtue on his outward parts.
How many cowards, whose hearts are all false
As stairs of sand, wear yet upon their chins
The beards of Hercules and frowning Mars,
Who, inward search'd, have livers white as milk. (85-90)
The phrase "false / As stairs of sand" is a simile to further illustrate the dishonesty of outward appearances. Naturally, stairs made of sand are not reliable; stairs made of sand would of course crumble under the weight of a person's foot. Shakespeare uses the simile of a heart being as untrustworthy as "stairs of sand" to depict the heart of a "coward." A coward is a person who lacks any courage in the face of difficulties; therefore, a coward's heart would crumble in the face of adversity just like stairs made of sand would crumble under the weight of someone's foot. However, a coward may also seem to be brave on the outside, and one can only tell the true nature of his/her cowardice when he/she is put to the test. Therefore, the "stairs of sand" simile also ties back to Bassanio's theme of untrustworthy appearances.
The phrase "livers white as milk" is another simile pertaining to lack of bravery. In these next two lines, Shakespeare uses the allusions of Hercules and Mars to depict men who seem to be brave. Hercules, the son of Zeus and the woman Alcmene, was a classic Greek hero known for his strength and success in adventures. Likewise, Mars was the Roman god of war, so he was also known for his strength. Bassanio caries his theme of untrustworthy appearances further by describing a coward, who looks like either Hercules or Mars, as also having a liver "white as milk." Liver is of course associated with health and was once also associated with fear, so one who became frightened also developed a weak, unhealthy liver; in fact, it can even be said it was believed that one who was easily frightened was frightened because the person's liver was weak and unhealthy. To describe a liver as being "white as milk" would be a means of describing a person as frightened. When we are scared, our faces turn white; Shakespeare would have associated the liver as turning white as well. Therefore, the simile is another simile to depict cowardice despite outwardly appearances.
The difference between the stair simile and the liver simile is that the first simile is used to paint the image of crumbling--a coward will emotionally crumble in the face of adversity. On the other hand, the second simile specifically depicts fear as opposed to any failing emotions.