Substance D
In this novel, Substance D is a fictional, metonymic symbol for all other kinds of mind-altering drugs. There isn't much mention of any other kinds of drugs in the novel, and Substance D is the primary subject, being the drug Arctor gets hooked on, as well as the most dangerous substance on the streets. Dick partially intended this novel to be a warning against the dangers of drugs, and Substance D is the target he invents to be a substitution for every type of addictive drug. As Substance D leads to the mental deterioration of Bob Arctor, it's clearly an invective against such substances and their abuses.
The Scramble Suit
Bob Arctor wears a scramble suit, a piece of technology that completely obscures his identity through visual projections and voice scramblers, while dealing with his government employers, for whom he is an undercover narcotics operative. At first, it appears that the only use of the suit is for disguising himself from those he works with. As the novel goes on, however, it becomes progressively clearer that the scramble suit is a symbol for Arctor's internal condition: he hides his own true identity from himself, and the drugs have so blurred his mind that he can no longer identify himself (for example, he doesn't recognize that he, Fred, is the same man as Bob, the man he sees on the surveillance footage).
German Poetry
As the novel moves toward its climax, Arctor's brain steadily deteriorates, and various pieces of evidence contribute to this conclusion. One of the most obvious ones is his random, sporadic regurgitations of German poetry, which he does in the middle of conversation with no apparent knowledge of having done so. The emergence of this poetry, then, comes to represent his fragmented mind, a terrible side effect of Substance D that blurs his reasoning faculties to begin to erase the distinction between past and present.
Connie
When Donna, Arctor's "girl," refuses to sleep with him on the grounds that she doesn't want to marry him because he's too ugly, Arctor leaves her place and returns to his house with Connie, a young female junkie, with whom he has sex. This act of selfish backlash against Donna has deeper symbolism, however; at the time, Arctor imagines that Connie is actually Donna, seeing her face superimposed on Connie's. When he goes back and watches the surveillance video of the incident (admittedly a bit weird), he notices that Connie actually transforms into Donna, making Arctor question his sanity. Connie is therefore a symbol of Donna as Arctor desires her - under his thumb and doing as he asks - while also being a symbol of Arctor's degenerating mental condition.
Jerry's Aphids
The novel opens with a scene where Jerry hallucinates that he's completely covered in hundreds of tiny bugs he insists are "aphids." Although they're obviously imaginary, he passes the illusion on to Charles Freck, who becomes suspicious and then frightened of these invisible beasts while "helping" Jerry collect them into a jar. These aphids are a symbol of the effects of hallucinogenic drugs; while they might seem harmless, they are easily spread and have long-lasting detrimental effects on a person's psychology (as seen by Freck's eventual suicide attempt).
