Not many people would first consult the Torah about such 21st-century developments as genetic modification, but it has much to say on the subject, according to Rabbi Shlomo Yaffe.

"The Torah is a blueprint of the proper utilization of the cosmos as a whole," said Yaffe, who will speak Thursday on "Designer Babies: Genetic Engineering from a Torah Perspective" at Chabad of San Mateo and Chabad of Greater South Bay in Palo Alto.

He will address genetic modification, a broad subject ranging from altering a gene structure that causes a life-threatening illness to cloning an entire human being. Taking into account the motivation for genetic modification as well as the health ramifications, Yaffe combines fundamentally Jewish principles with ideas from Jewish mysticism to answer questions these practices raise.

"We will discover there are very clear visions about these things," he said. "Judaism sees science as a useful tool to fulfill its agendas, provided it is controlled by the ethical heart and brain of Jewish law."

Yaffe, founder of the Institute for Jewish Literacy, an adult education center, leads the Orthodox Congregation Agudas Achim in West Hartford, Conn. His authority on genetic engineering comes not from a scientific background but from a Jewish one.

"My concern is the spiritual, religious and ethical implication of this issue," said Yaffe, a father of six who also teaches advanced Talmud and Judaic philosophy at the Hebrew High School of New England.

Some issues are simple.

"If you're really saving someone's life, a lot of the theoretical and spiritual concerns go out the window," he said.

Conversely, Judaism rejects the idea of creating a more valuable person through genetic engineering because it does not put a relative value on human life. A comatose 88-year-old man has the same worth as a 19-year-old boy, Yaffe said.

"Those are certainly two poles that are a clear black and white, but we need to know what the shadings are," he said. "I find a point in the gray and draw a line."

Even black and white can look gray. Jewish law, Yaffe said, can be very permissive when issues boil down to saving a life. For example, anyone with a life-threatening illness — or aiding someone with a life-threatening illness — can break Shabbat.

"We are children playing the control room of a nuclear reactor," Yaffe said of cloning. "We now have the capacity to have incredible control over the universe. We had better grow up morally and ethically very quickly."

For guidance Yaffe looked for broad ideas about present benefit vs. future danger, about leaving well enough alone vs. making changes and about what degree of risk is acceptable for a benefit that enhances rather than saves a life.

He turned to various Judaic readings, including the Kuzari, a 12th-century philosophical text written by Rabbi Yehudah Halevi, which addresses the human status of artificial life. Halevi envisioned the concept of creating a human from a microscopic level, and he argued that a body created by man automatically receives a human soul.

Yaffe also drew from commentaries that address the moral and ethical issues of hybridization, including the Talmud, codes of Jewish law and works of Kabbalah.

"There's a lot of discussion of doing things that can create environmental or health damage," he said.

Scientific developments like genetic engineering interest Yaffe in part because he believes in the Chabad principle that the spiritual and physical worlds mirror each other perfectly. The latter, he said, must be addressed aggressively and thoroughly from a Torah perspective.

"It's an intrinsic part of what it means to be a Jew," he said.

Yaffe, who has given the lecture four times on the East Coast, said most people tell him they never realized the depth and sophistication of Judaism. Furthermore, people seem reassured by the guidance.

"People are comforted to know that as powerful is our ability to change things, we have an equally powerful guiding source," he said. "I think people came away encouraged with a new appreciation of what they can find in their own heritage."